Generally speaking, you should use only complete sentences in formal writing, which includes the writing you do in college. However, writers often use sentence fragments intentionally. (I often use the fragment “Like so” on this blog.) Here is a short discussion, con and pro, of sentence fragments that appeared on the New York Times web site:
For a sentence to be a sentence we need a What (the subject) and a So What (the predicate). The subject is the person, place, thing or idea we want to express something about; the predicate expresses the action, condition or effect of that subject. Think of the predicate as a predicament — the situation the subject is in.
I like to think of the whole sentence as a mini-narrative. It features a protagonist (the subject) and some sort of drama (the predicate): The searchlight sweeps. Harvey keeps on keeping on. The drama makes us pay attention.
When a sentence lacks one of its two essential parts, it is called asentence fragment. Like the flotsam I mentioned earlier, fragments are adrift, without clear direction or purpose.
Playing with sentence fragments can be fun — the best copywriters use them for memorable advertising slogans (Alka-Seltzer’s “Plop plop, fizz fizz”). But there are plenty of competing Madison Avenue slogans to convince you that a full sentence registers equally well — from Esso’s “Put a tiger in your tank” to The Heublein Company’s “Pardon me, would you have any Grey Poupon?” While sentence fragments can be witty, they are still shards of thoughts, better suited to hawking antacids than to penning the Great American Novel or earnestly attempting to put inchoate thoughts into indelible words.
The Sentence as a Miniature Narrative
By CONSTANCE HALE
March 19, 2012, 9:30 PM
I’d like to make a case for the sentence fragment, though. In all kinds of business writing (reports, proposals, blogs, articles), fragments add punch. After medium to long sentences explaining something complex, a fragment can introduce a twist or wake up the reader with a bold comment sandwiched between white space (which the eye welcomes too). Used sparingly, it’s a dandy addition to the writer’s toolkit.
Linda McDaniel – comment left in reponse