A sentence should express a complete thought, and a complete sentence should include a subject and a verb. When one or the other is missing, we are left with a sentence fragment. In ordinary conversation we use fragments all the time, and occasionally, for special emphasis, a writer may use a fragment. But for academic writing, we should always write complete sentences.
1) Make sure every sentence has both a subject and a verb.
Example: Babies require a great deal of attention.
2) Watch out for -ing words. No word ending in -ing can ever be the complete verb of a sentence.
Example: Fox Mulder running down the street.
Correct: Fox Mulder is running down the street.
3) A group of words containing a subject and a verb is called a clause. In the English language there are two types of clauses:
* Independent: This type of clause has a subject and a complete verb, and can stand alone as a complete sentence.
* Dependent: This type has a subject and a verb, but it cannot stand alone because it begins with a word or word group such as:
although, because, since, though, if, as if, where, unless, as soon as, whereas, in order that, when, whenever, while, before, after, as, until, so that, as long as, such as, providing that, during.
The following are dependent clauses, not complete sentences:
Because he dropped his cell phone.
Providing he asks nicely.
4) When a dependent clause comes at the beginning of a sentence, it should followed by a comma.
When he spoke, the audience was silent.
While she was gone, I fed her dog and fish .
However, if the independent clause comes first, you may or may not need a comma. Use a comma only if it is necessary to make the meaning clear.
The audience was silent when he spoke.
I fed her dog and fish while she was gone.