https://www.teacherspayteachers.com/Product/7-deadly-sins-of-grammar-954207

its v. it’s. Many of us may have learned our grade-school grammar too well. “Apostrophe s” is the sign for possession, right? So when an it owns something, we write it’s. But it’s is a contraction of it is. 

they v. he or she. He or she is cumbersome when you don’t know a person’s gender. We used to use the masculine he. Modern feminism made that unpalatable. Many writers try to be politically correct, using they, and end up grammatically incorrect. If gender is unknown, you have three good choices: 1) use he or she; 2) pick he in some instances, she in others; 3) make the antecedent plural and use they. (Instead of “a person must speak his or her mind”

between you and I. Between is a preposition, and prepositions must be followed by objects. 

who v. whom. Who is a pronoun we use for the subject of a sentence, as in “Who called?” Whom is the pronoun we use for the object of a sentence, as in “You called whom?”

good v. well. How many times have you heard a sentence like “This car runs good”? Get this straight: Good is an adjective; it modifies a noun. Well is an adverb; it modifies a verb, adjective, or other adverb.   

fewer v. less. Fewer is the correct adjective when the noun it modifies is a plural comprising multiple units. Less is the correct adjective when the noun it modifies is something that is a mass, or an idea, rather than a number of units. Nonfat milk has fewer calories than whole milk; we should have less Coke in our diet than milk.

lay v. lie. Learn this to stay a step ahead of most writers and editors. Lay is a transitive verb. It must have an object to complete its meaning: A chicken lays eggs. Lie is an intransitive verb. It needs no object to make sense: The dog lies down. (Down is an adverb.)