Sentence types

(Source: scientificpsychic.com)
English has four main sentence types:

1. Declarative Sentences are used to form statements.
   Examples: "Mary is here.", "My name is Mary."
2. Interrogative Sentences are used to ask questions.
   Examples: "Where is Mary?", "What is your name?"
3. Imperative Sentences are used for commands.
   Examples: "Come here.", "Tell me your name."
4. Conditional Sentences are used to indicate dependencies between events or conditions.
   Example: "If you cut all the trees, there will be no forest."
   

It is not polite to use imperative sentences without the word "Please". It is better to say "Please, come here." than just "Come here." The simplest English sentences are imperative sentences with a single verb (Example: "Help!").

Sentences may be simple or compound. Compound sentences consist of two or more simple sentences joined by conjunctions.
Examples:
"Come here and sit down.",
"My name is Mary and I live in New York.",
"What is your name and where do you live?",
"Either John will go to New York or Mary will come here."

It is not correct to combine different sentence types in a compound sentence.
Wrong: "Mary lives in New York and come here."
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<English Sentence> =
       <Simple Sentence> |
       <Compound Sentence>

<Simple Sentence> =
       <Declarative Sentence> |
       <Interrogative Sentence> |
       <Imperative Sentence> |
       <Conditional Sentence>

<Compound Sentence> =
       <Simple Sentence> <conjunction> <Simple Sentence> |
       "Either" <Declarative Sentence> "or" <Declarative Sentence> |
       "Either" <Imperative Sentence> "or" <Imperative Sentence>

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(Source: learningnerd.wordpress.com)

Types of Sentences by Structure

• Simple Sentence – “I love chocolate.” One independent clause (underlined).

• Compound Sentence - “I love chocolate, and I love eating chocolate.” Two or more independent clauses.

• Complex Sentence – “I love chocolate because it’s decadent.” One independent clause and one or more dependent clauses (italicized). Note: according to Wikipedia, a sentence like “The dog chewed up the shoes that I just bought” is a simple sentence, not a complex sentence, because the relative clause “that I just bought” simply modifies the noun without performing any other function. I’m not sure how accurate this is, however.

• Complex-Compound Sentence – “I love chocolate because it’s decadent, and I love eating chocolate because it’s delicious.” Two or more independent clauses and one or more dependent clauses.

Those four categories apply to normal, grammatical sentences. However, some of our most common expressions are sentences that don’t follow the rules — see Major and Minor Sentences.
Note: obsessive syntacticians (is there any other kind?) have also named more specific types of sentences, which I’ll address when I start learning about the finer points of writing style.

Types of Sentences by Purpose

• Declarative Sentence – “I love chocolate.” Used to make a simple statement. Most sentences are declarative.

• Interrogative Sentence – “Do you love chocolate?” Used to ask a question. See also Rhetorical Question. 

• Exclamatory Sentence – “I need chocolate!” Used for emphasis and emotion.

• Imperative Sentence – “Please buy me some chocolate.” Used for commands, with the pronoun you always implied.

• Conditional Sentence – “If I had a billion dollars, I would buy a castle made of chocolate.” Used to express what one would do if a condition were met. There are several types of conditional sentences: the present general (or zero condition), the future more-vivid (or first condition), the future less-vivid (or second condition), the present contrafactual (also sometimes called the second condition), and the past contrafactual (or third condition).