Sentence structure
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.
Greg thinks he knows stuff. This is a complete sentence as it contains a verb (thinks), expresses a complete idea and it does not need any further information for the reader to understand the sentence.
When Greg thinks is an incomplete sentence. It contains a verb, but the opening word when tells us that something happens when Greg thinks; we need more information to complete the idea. When Greg thinks, he likes to type. This is now a complete sentence, as the whole idea of the sentence has been expressed. There is another theory. Which should not be ignored. There is another theory that should not be ignored, Greg thinks he knows stuff other people don't know.