1) A verb — a word or phrase expressing action or state of being
2) A subject — who or what you’re talking about in the sentence
3) A thought that makes sense by itself, that’s . . . well, complete.

Here are some examples of complete sentences and incomplete sentence fragments, all from a (non-existent) essay answering the real application question “Describe a situation in which you took a stand that was unpopular with the majority”:

Incomplete fragment: Because I had the guts to defy Mr. Pickle.

Why it’s incomplete: You’ve given a reason, but it’s not attached to any context. True, the context may be clear from the information in other sentences, but this one can’t stand alone.

Complete sentence: I didn’t drop out of chemistry when I was assigned 149 extra laboratory reports as punishment, because I had the guts to defy Mr. Pickle.
Why it’s complete: Now you have a thought that ends in a logical place. Nothing is flapping in the breeze, waiting for more words.

Another fragment: Mr. Pickle, who was overly fond of vinegar and often sang “The Brine Song” at the beginning of class, regardless of how much work we were supposed to accomplish.

Why it’s incomplete: The “who” signals a description, but there’s no main idea — no verb, in grammatical terms — to match with “Mr. Pickle.” The reader has a description but is waiting for the central statement to conclude.

Complete sentence: Mr. Pickle was overly fond of vinegar and often sang “The Brine Song” at the beginning of class, regardless of how much work we were supposed to accomplish.

Why it’s complete: When you extract the “who,” the reader stays on one track. The verb “was” matches “Mr. Pickle.” The meaning comes across as finished.

Another complete sentence: Mr. Pickle, who was overly fond of vinegar and often sang “The Brine Song” at the beginning of class, regardless of how much work we were supposed to accomplish, eventually drowned in a vat of cucumber salad.

Why this one’s complete: Now when the “who . . . accomplish” description is extracted, a complete thought remains: “Mr. Pickle eventually drowned in a vat of cucumber salad.”
One more thing about complete sentences: Don’t stick two of them together without any glue. The “glue” in Grammar World is a joining word (and, or, nor, but, for, yet, because, since, after, although, and so on) or a semicolon (;). If you attach one complete sentence to another, be sure you’ve got one of those “glue” words, also known as conjunctions. Some examples:

Illegal joining: I told the class that Mr. Pickle’s plan was immoral, the class didn’t listen to me.

Why it’s illegal: Check out the stuff before the comma. It’s a complete thought with a subject and a verb. So is the stuff after the comma. Hence you have two complete sentences, joined only by a comma. That’s a no-no.

Legal joining: I told the class that Mr. Pickle’s plan was immoral, but the class didn’t listen to me.

Another legal joining: I told the class that Mr. Pickle’s plan was immoral; the class didn’t listen to me.

Some words look like “glue,” but they aren’t. (Think of these words as the stick-um on very old envelopes. It doesn’t attach anything; it just looks good.) Common “false glue” words include then, also, moreover, consequently, however, and nevertheless. If you want to attach one complete sentence to another with one of these words, add a semicolon, as in this example:

Mr. Pickle was forced to destroy his Purple Bomb; consequently, the world was saved, but I got extra chem homework.

Punctuation

Seeing spots before your eyes? How about wiggly lines? Don’t call the eye doctor yet; you’re probably just having a punctuation meltdown. Punctuation — the periods, commas, apostrophes, and other symbols in your writing — serve a valuable purpose. Punctuation indicates how words should be grouped, where the pauses are, and who said what. All these terrific accomplishments go hand in hand with one inescapable fact: The rules of punctuation are arbitrary (The comma goes there because I said so!) and annoying (Who cares where the comma goes?). Nevertheless, you have to follow these dumb rules in your admission essay. No doubt the admission committee wants to see whether you are capable of following dumb rules, so they’ll know whether you’re likely to follow their dumb rules after you’re admitted.

Endmarks

Endmarks — periods, question marks, and exclamation points — come at the end of a sentence. (How do grammarians come up with this terminology?) Some rules:

1) Don’t place two endmarks at the end of a sentence.
2) If the sentence is a statement ending with a quotation, place the period inside
the quotation marks, as in this example:
3) My guidance counselor said that I am “Hahvah material.”
4) For a sentence ending with a quotation, use question marks or exclamation points in this way: Place the question mark or exclamation point inside the quotation marks if the quoted words are a question or an exclamation. Please the question mark or exclamation point outside if the entire sentence, but not the quoted words, is a question or an exclamation. Some examples:
5) Was the guidance counselor correct in calling me “Hahvah material”?
6) I ask you, “Am I Hahvah material?”