1. Declarative, Interrogative, Imperative, and Exclamatory Sentences

According to their use, sentences are declarative, interrogative, imperative, or exclamatory. A declarative sentence makes an assertion or a statement. An interrogative sentence asks a question. An imperative sentence expresses a command or a request. An exclamatory sentence expresses a strong feeling or emotion, such as surprise, pain, or joy.

2. Simple, Compound, Complex, and Compound-Complex Sentences

According to their structure, sentences are simple, compound, complex, or compound-complex.
A simple sentence has only one subject and one predicate-verb, but it may contain more than one object, attribute or adverbial. A sentence with two or more subjects or predicate-verbs like "He and his brother went to town and bought a pile of books" is called a simple sentence in some grammar books and a compound one in others. Short simple sentences are usually emphatic; they have special clarity, and provide variety when used with longer sentences.
A compound sentence consists of two or more independent clauses (or simple sentences) related to each other in meaning, and linked by a coordinating conjunction (and, but, or, etc.) or by a semicolon without a conjunction. Coordinated ideas should be compatible and roughly equal in importance, or take shape one by one in orderly sequence.
A complex sentence contains one main (or principal) clause and one or more dependent (or subordinate) clauses, with a connective word denoting the relation between the two parts. The dependent clause may play the part of a subject, an object, a predicative, an attribute, or an adverbial in the main clause. As a rule, the major idea is expressed in the main clause and the idea or ideas of lesser importance in the subordinate clauses.
A compound-complex sentence contains at least two main clauses and at least one dependent clause - a combination of a compound and a complex sentence.
Examine the following examples:

    To Americans, industriousness, thrift, and ambition are positive values. We encourage our children to be competitive, to get ahead, to make money, to acquire possessions. In games and in business alike the aim is to win the game, the trophy, the contract. We go in for labor-saving devices, gadgets, speed, and short-cuts. We think every young couple should set up a home of their own, and we pity the couple who must share their home with a parent, let alone with other relatives. Actually, of course, not all Americans hold all these values, and those who do may hold other, and at times contradictory, values that affect their ways of behaving. In the main, however, the collective expectation of our society is that these are desirable goals, and the individual, whatever his personal inclination, is under considerable pressure to conform.

- Ina Corinne Brown

Our so-called best schools [of city slums] are turning out students most of whom, in any real and important sense, are as inarticulate as the most deprived children of the ghettos, as little able to speak or write simply and directly about things of importance to them, what they know, want, and care about. The training in writing that they get, unless they are very lucky, is largely training in bullslinging and snowjobbery. Every year students at all levels write millions of papers. It is a safe bet that most of the time - I would guess over 95 percent - the writers of these papers do not care about and in fact have no honest and genuine opinions about what they are writing, and would not write it if they were not made to. I once asked a very able high school senior, a straight A student in English, if she ever kept any of her old English papers. She looked at me amazed. "For heaven's sake," she said, "What for?"

- John Holt

In each of the above two passages are used various types of sentences, from simple to compound-complex. Variety in sentence structure is generally required.
Short simple sentences are often used to make emphatic or important statements, such as the first sentence of the first passage and the last two sentences of the second passage.
Long complex sentences express complex ideas clearly and accurately, for they have room for all kinds of modifiers. The last two sentences of the first passage and the first and third sentences of the second are good examples.

3. Loose, Periodic, and Balanced Sentences

From a rhetorical point of view, sentences are loose, periodic, or balanced. A loose sentence puts the main idea before all supplementary information; in other words, it puts first things first, and lets the reader know what it is mainly about when he has read the first few words. The reverse arrangement makes a periodic sentence: the main idea is expressed at or near the end of it, and it is not grammatically complete until the end is reached. The reader does not know what it is mainly about until he finishes reading it. Compare:

She decided to study English though she was interested in music.
Although she was interested in music, she finally decided to study English.

The main idea of both sentences is the fact that she decided to study English. This idea is put at the beginning of the first sentence and at the end of the second, thus making one a loose sentence and the other a periodic one. Besides, the first part of the first sentence is complete in structure, but that of the second is only an adverbial clause and cannot be called a sentence without the second part.

    It is a truth universally acknowledged, that a single man in possession of a good fortune must be in want of a wife.

- Jane Austen

This is the sentence with which Jane Austen begins her Pride and Prejudice. It is clearly of the periodic structure because neither in syntax nor in meaning is it complete before the last word. The sentence follows a climactic order, the last word being the the most important, and because many words are piled up before the key word, the sense of climax is made very strong.
Here is the opening paragraph of Joseph Conrad's Lord Jim:

    He was an inch, perhaps two, under six feet, powerfully built, and he advanced straight at you with a slight stoop of the shoulders, head forward, and a fixed-from-under stare which made you think of a charging bull. His voice was deep, loud, and his manner displayed a kind of dogged self-assertion which had nothing aggressive in it. It seemed a necessity, and it was directed apparently as much at himself as at anybody else. He was spotlessly neat, apparelled in immaculate white from shoes to hat, and in the various Eastern ports where he got his living as ship-chandler's water-clerk he was very popular.

Most of the sentences are compound and compound-complex, and all of them, except the second half of the last sentence, are loose in structure. There is no suspense or climax; the tone is easy, relaxed and informal. The writer uses these loose sentences because he is enumerating facts or ideas of equal importance, and also because he aims at a natural and orderly presentation.
Loose sentences are easier, simpler, more natural and direct; periodic sentences are more complex, emphatic, formal, or literary.
Here are more examples of the two types:

    The sentence you are now reading is so constructed that the completion of its meaning and effect depend on reading it to the very last word. Such a sentence is called periodic, as contrasted with the other type, loose, of which this is an example - a sentence that might be stopped at several points (in this sentence, after periodic, type, loose, example, or points).

When a sentence contains two or more parts of the same form and grammatical function, it is one with parallel constructions:

Let us be ruthless in our criticism, cruel to personal vanities, indifferent to age, rank or experience if these stand in our way.

- Norman Bethune

It does not do to live in memories, in regrets for the good old days, or in sadness about friends who are dead. One's thoughts must be directed to the future, and to things about which there is something to be done.

- Bertrand Russell

The feeling of the nation must be quickened; the conscience of the nation must be roused; the propriety of the nation must be startled; the hypocrisy of the nation must be exposed; and its crimes against God and men must be proclaimed and denounced.

- Frederick Douglass

Such parallel sentences are emphatic and forceful. When a sentence contains two parallel clauses similar in structure but contrasted in meaning, it is a balanced sentence.

On hearing the news, he was angered, and I was saddened.
The politician is concerned with successful elections, whereas the statesman is interested in the future of his people.
In Plato's opinion man was made for philosophy; in Bacon's opinion philosophy was made for man.

- Thomas Babington Macaulay

Balanced sentences are impressive because of the contrast, and pleasing to hear because of the rhythm. They are mainly used in formal writing, like expository and argumentative prose, and speeches.

4. Short and Long Sentences

Short sentences are usually emphatic, whereas long sentences are capable of expressing complex ideas with precision, because it may contain many modifiers. Short sentences are suitable for the presentation of important facts and ideas, and long sentences for the explanation of views and theories, or the description of things with many details.
Look at the following passage which makes good use of short sentences:

    Our city is on the threshold of a great era. Of this we can be sure. But in order to fulfill the promise of the future, we must be willing to work - and to spend. We need a larger police force for public protection. We need a modernized fire department. We need an enlarged library. We need to increase the pay scale of teachers in the public schools. We need to improve our water supply. For all these needs there is but one solution. We must see that the bond issue is approved by the voters in the November election.

Here each short sentence puts forward an important suggestion. If the points were grouped into two or three longer sentences, there would not be the force and clarity of these short sentences.
The following passage describes how a man saved a drowning girl:

    He crouched a little, spreading his hands under the water and moving them round, trying to feel for her. The dead cold pond swayed upon his chest. He moved again, a little deeper, and again, with his hands underneath, he felt all around under the water. And he touched her clothing. But it evaded his fingers. He made a desperate effort to grasp it....
    He laid her down on the bank. She was quite unconscious and running with water. He made the water come from her mouth, he worked to restore her. He did not have to work very long before he could feel the breathing begin again in her; she was breathing naturally. He worked a little longer. He could feel her live beneath his hands; she was coming back. He wiped her face, wrapped her in his overcoat, looked round into the dim, dark gray world, then lifted her and staggered down the bank and across the fields.

- D. H. Lawrence

The many short sentences in the above passage vividly describe the man's rapid movements and make the reader feel the tension the man was experiencing at the moment.
Long sentences are common in legal, political and theoretical writing, which depends on modification for accuracy.

Art, in the sense here intended - that is, the generic term subsuming painting, sculpture, architecture, music, dance, literature, drama, and film - may be defined as the practice of creating perceptible forms expressive of human feeling. I say "perceptible" rather than "sensuous" forms because some works of art are given to imagination rather than to the outward senses. A novel, for instance, usually is read silently with the eye, but is not made for vision, as a painting is; and though sound plays a vital part in poetry, words even in poetry are not essentially sonorous structures like music....
"Feeling" as I am using it here covers much more than it does in the technical vocabulary of psychology, where it denotes only pleasure and displeasure, or even in the shifting limits of ordinary discourse, where it sometimes means sensation (as when one says a paralyzed limb has no feeling in it), sometimes sensibility (as we speak of hurting someone's feelings), sometimes emotion (e.g., as a situation is said to harrow your feelings, or to evoke tender feeling), or a directed emotional attitude (we say we feel strongly about something), or even our general mental or physical condition, feeling well or ill, blue, or a bit above ourselves. As I use the word, in defining art as the creation of perceptible forms expressive of human feeling, it takes in all those meanings; it applies to everything that may be felt.

- Susanne K. Langer

The writer is trying to define such abstract terms as "art" and "feeling", and it is necessary for her to use so many words to make the definitions accurate and prevent misunderstanding.
In fiction long sentences are sometimes used to describe a person, a thing or a scene.

Mrs. Chalmers was kind of fat and her hair was pretty blond and her complexion was soft and pink and she always looked as though she had been in the beauty parlor all afternoon. She always said "My, you're getting to be a big boy" to Peter when she met him in the elevator, in a soft voice, as though she was just about to laugh. She must have said that fifty times by now. She had a good, strong smell of perfume on her all the time, too.
Mr. Chalmers wore pince-nez glasses most of the time and he was getting bald and he worked late at his office a good many evenings of the week. When he met Peter in the elevator he would say, "It's getting colder," or "It's getting warmer," and that was all, so Peter had no opinion about him, except that he looked like the principal of a school.
But now Mrs. Chalmers was on her knees in the vestibule and her dress was torn and she was crying and there were black streaks on her cheeks and she didn't look as though she'd just come from the beauty parlor. And Mr. Chalmers wasn't wearing a jacket and he didn't have his glasses on and what hair he had was mussed all over his head and he was leaning against the Early American wallpaper making this animal noise, and he had a big, heavy pistol in his hand and he was pointing it right at Mrs. Chalmers.

- Irwin Shaw

Of the eight sentences in the above passage six are long, and they give detailed descriptions of the two characters. Were they broken into many short sentences, the contrast between the normal Chalmers couple and the couple on that particular occasion would not be so striking.
Various sentence structures have been discussed. The basic principle is that the structure should fit the idea being expressed. In other words, the idea determines the choice of the structure, not the other way round.