For your highest SAT Writing score, be sure that you not only understand, but are also actively looking for, SAT questions that deal with these issues.
1. Noun – Verb Agreement: a verb must agree in number (singular vs. plural) with the noun to which it belongs, not with a word or phrase in between. Find this error by putting a box around the Subject and the Predicate and checking for agreement.
2. Parallel Structure: Items linked by a conjunction such as “and” must all have the same grammatical structure. Incorrect Example: “I like running, biking, and to swim.”
3. Pronoun Antecedent: A pronoun must refer to a particular noun in the sentence and must agree in number and type with that noun. Typically “it” or “they” is misused.
4. Introductory Modifier: If a sentence does not begin with a noun phrase, it usually begins with an introductory phrase (usually a phrase attached to the beginning of a sentence using a comma) must modify the noun that follows the comma, not a noun later in the sentence. These are hard to notice unless you are looking for them.
5. Verb Tense: All verbs in a sentence must be conjugated in the same tense unless intentionally referring to different time frames.
6. Comma and Semicolon Usage: A semicolon is only used to join together two independent clauses (and can be replaced by a period), while a comma can join two of these only when paired with a conjunction.
7. “One”-“You” Switcheroo: If the pronoun “one” or “you” is used, stick with the same pronoun throughout the entire sentence. If you EVER see the word “one”, look for this error.
8. Conjunction Combinations: “Neither…nor”, “either…or”, “not only…but also”, “between…and”, “no sooner…than” : These phrases operate only in pairs – that is, “neither” can never be used in a sentence without also using the word “nor”.
9. Wrong Preposition: Nouns and prepositional phrases must use the proper (idiomatic) preposition. Errors are more obvious to the ear than the eye. These can be extremely hard to notice. Examples: “preoccupation with”, “in contrast to” “prefer…over”, “protest…against”.
For more details on these issues, see 11 Rules of Writing, Grammar, and Punctuation