What is a complete sentence?
Sentences provide us with the framework for the clear written expression of our ideas. The aim in writing is always to write in complete sentences which are correctly punctuated. Sentences always begin with a capital letter and end in either a full stop, exclamation or question mark. A complete sentence always contains a verb, expresses a complete idea and makes sense standing alone.
Andy reads quickly. This is a complete sentence as it contains a verb (reads), expresses a complete idea and it does not need any further information for the reader to understand the sentence.
When Andy reads is an incomplete sentence. It contains a verb, but the opening word when tells us that something happens when Andy reads; we need more information to complete the idea.
When Andy reads, he reads quickly. This is now a complete sentence, as the whole idea of the sentence has been expressed. The following examples show the incomplete sentences in italics.
There is another theory. Which should not be ignored.
There is another theory which should not be ignored.
The proposal was finally rejected. Although they considered it.
Although they considered the proposal, it was finally rejected.
To check that you are writing in complete sentences, try reading your sentences aloud, pausing as indicated by the punctuation. Can each sentence stand alone as a complete thought? If further information is needed to complete the idea, then it is not a complete sentence.