A sentence is a sequence of words constructed in accordance with the rules of grammar. Such a group has a sense of completeness and clarity of meaning. It will be constructed around at least one noun phrase acting as the subject of a finite verb, i.e. it will contain at least one main clause The rules of grammar mainly concern the order words must take in a sentence, technically called its syntax and, to a lesser extent, the form words must take, called their morphology.

Sentence 1) below shows standard syntax and morphology (i.e. standard grammar):
1). 'The cat sat on the mat.'

Sentence 2) shows non-standard morphology:
2). 'The cat sitted the mat on.'

Sentence 3) shows non-standard syntax:
3). 'The cat on the mat sat.'

A group of words that is a sentence is made obvious to the eye (i.e. in writing) by an opening capital letter and a final full stop, question mark or exclamation mark. It is made obvious to the ear (i.e. in speech) by the use of pauses. It is made obvious to the mind because it makes sense alone.

A sentence may loosely be said to be a coherent group of words that expresses a single complete thought about something (or someone).

A sentence can be one of three main types:

1. A simple sentence is a sentence that contains a single subject and verb, i.e. an independent clause.

2. A compound sentence is a sentence that contains more than one main clause. These clauses must be linked by co-ordinating conjunction or a semicolon.

3. A complex sentence is a sentence that contains a mixture of clause types. A complex sentence must contain (as all sentences) at least one main clause but will also contain a second kind of clause acting as a dependent or subordinate clause. Subordinate clauses often begin with a subordinating conjunction such as however, although, even though, because, etc. There is also a special kind of sentence, often used in speech, called a 'minor sentence'.

A sentence can fulfil one of four functions:

1. It can make a statement. This is called a declarative sentence, e.g. 'I am overweight.' Declaratives usually follow the word order SV (subject first, verb second)

2. It can ask a question. This is called an interrogative sentence, e.g. 'Am I overweight?' and indicated by a question mark. Interrogatives usually follow the word order VS (verb first, subject second)

3. It can demand an action. This is called an imperative sentence, e.g. 'Sit down, please.' indicated by a lack of subject (but 'you' is implied).

4. It can make an exclamation. This is called an exclamatory sentence, e.g. 'What a mess!', indicated by an exclamation mark.

» Linguistic Library (Mike Green)