Most people recognise a sentence as a unit which begins with a capital letter and ends with a full stop (period), a question mark, or an exclamation mark. Of course, this applies only to written sentences. Sentences have also been defined notionally as units which express a "complete thought", though it is not at all clear what a "complete thought" is.
It is more useful to define a sentence syntactically, as a unit which consists of one or more clauses. According to this definition, the following examples are all sentences:
Sentence  is a SIMPLE SENTENCE -- it contains only one clause.
Sentence  consists of a matrix clause You can borrow my pen if you need one, and a subordinate clause if you need one. This is called a COMPLEX SENTENCE. A complex sentence is defined as a sentence which contains at least one subordinate clause.
Finally, sentence  consists of two clauses which are coordinated with each other. This is a COMPOUND sentence.
copyright The Survey of English Usage 1996-1998
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