Every sentence completion question has at least one clue. In fact, the writers of these types of SAT problems must put clues into the sentences; otherwise, there would be no way for you, the test-taker, to know word(s) the writers want. Consider the following sentence:
My sister Natalie is very _____________.
That sentence would be a very poor sentence to use as a Sentence Completion problem, because there’s not enough information in the sentence itself for you to know what word the SAT writers have in mind. Your sister Natalie could virtually be anything: funny, happy, angry, silly, ruthless, garrulous (look it up), pedantic (look it up), or apathetic (look it up).
Contrast the previous example with this one:
Although my brother Brad is a generous person, my sister Natalie is very _____________.
This example, although perhaps slightly simplistic, exemplifies most Sentence Completion sentences. Notice that this time, you’re given clues that lead you to figure out what type of word belongs in the blank. You have two clue words here: although and generous. The word although signals that you should look for the opposite of a certain word—in this case, generous. So you should realize that the correct answer should mean “the opposite of generous.” What is a word that means that? How about selfish? Then, look at the five answer choices. Eliminate each word that doesn’t mean selfish. Keep any answer choices in which you don’t know the word. Then, look at the remaining answer choices. Is there one that you know means “the opposite of generous” or selfish? If yes, you’ve found your answer. If you know none of the remaining words, then guess and continue to the next question.
So, with Sentence Completion problems, your main jobs are to read the sentence, spot the clues, and to come up with a word (or phrase) that exemplifies what should belong in the blank(s).