Do It = Wrong; Do So = Right

If it is one of the most dangerous words on the Writing section of the SAT, then do it is one of the most dangerous phrases. While Im hesitant to say that any particular word or phrase will be wrong 100% of the time the College Board can always figure out a new way to test a particular rule do it comes pretty close. The reason this phrase is so often incorrect is based on the fact that pronouns such as it can only refer to nouns or gerunds ( the -ING form a verb), never to any other part of speech such as verbs or adjectives. For example: Incorrect: People who become vegetarians often do it because they believe strongly in animal rights. What does the word it refer to? Becoming vegetarians. But the gerund becoming doesnt actually show up in the sentence, only the verb become. And a pronoun can only refer to a noun or a gerund, not a verb. The pronoun it therefore doesnt actually refer to anything (in grammatical terms, we can say that it lacks an antecedent), and the sentence cannot be correct. To fix it, we must instead say: Correct: People who become vegetarians often do so because they believe strongly in animal rights. On the flipside, anytime you see the phrase do so underlined, it is almost certainly correct, and theres a pretty good chance that the answer to the question will be No error.