Of all the types of verbal questions, Sentence Correction is the most like math: they are usually short, and the test a finite number of rules.

Sentence correction section

Just like the quantitative section, GMAT Sentence Correction doesn't test you on every grammar rule, or on the entirety of the English language.

It focuses on a few rules, and expects that you know them inside and out.

Most SC questions hinge on a small number of grammatical concepts:

Subject/verb agreement

Modifiers

Parallelism

"Effective expression

Idioms

Sentence correction rules

IT’S/ITS
“Its” is the possessive form of the pronoun it.

This is unique because usually an apostrophe is needed to show possession. Its’ is never correct. It’s is the contraction for “it is” or “it has.”

WHOSE/WHO’S
“Who’s” is a contraction of “who is” or “who has.” “Whose” is the possessive form of the pronoun “who.” If “who’s” cannot logically replace “who is” or “who has.”

AMONG/BETWEEN
“Among” is used when dealing with a group of three or more. “Between” can only be used for a group of two.

AMOUNT/NUMBER
“Amount” can only refer to an uncountable noun. We can use “number” to refer to a countable noun.

FEWER/LESS
“Less” refers to an uncountable noun, whereas “fewer” refers to a countable noun.

OVER/MORE THAN
“Over” refers to location only and is a preposition. “More than” is only used to refer to a number or a quantity.

MUCH/MANY
“Much” refers to an uncountable noun. “Many” refers to a countable one.

BETTER/BEST
This concept tests the comparative/superlative forms. Comparative forms like “better”, “more”, etc. are only used when two things are being compared. Superlative forms like “best”, “most”, etc. are used to compare three or more things.