20 Common Grammar Mistakes That (Almost) Everyone Makes

From Litreactor:

Below are 20 common grammar mistakes I see routinely, not only in editorial queries and submissions, but in print: in HR manuals, blogs, magazines, newspapers, trade journals, and even best selling novels. If it makes you feel any better, I’ve made each of these mistakes a hundred times, and I know some of the best authors in history have lived to see these very toadstools appear in print. Let’s hope you can learn from some of their more famous mistakes.

Who and Whom
This one opens a big can of worms. “Who” is a subjective — or nominative — pronoun, along with “he,” “she,” “it,” “we,” and “they.” It’s used when the pronoun acts as the subject of a clause. “Whom” is an objective pronoun, along with “him,” “her,” “it”, “us,” and “them.” It’s used when the pronoun acts as the object of a clause. Using “who” or “whom” depends on whether you’re referring to the subject or object of a sentence. When in doubt, substitute “who” with the subjective pronouns “he” or “she,” e.g., Who loves you? cf., He loves me. Similarly, you can also substitute “whom” with the objective pronouns “him” or “her.” e.g., I consulted an attorney whom I met in New York. cf., I consulted him.

Which and That
This is one of the most common mistakes out there, and understandably so. “That” is a restrictive pronoun. It’s vital to the noun to which it’s referring. e.g., I don’t trust fruits and vegetables that aren’t organic. Here, I’m referring to all non-organic fruits or vegetables. In other words, I only trust fruits and vegetables that are organic. “Which” introduces a relative clause. It allows qualifiers that may not be essential. e.g., I recommend you eat only organic fruits and vegetables, which are available in area grocery stores. In this case, you don’t have to go to a specific grocery store to obtain organic fruits and vegetables. “Which” qualifies, “that” restricts. “Which” is more ambiguous however, and by virtue of its meaning is flexible enough to be used in many restrictive clauses. e.g., The house, which is burning, is mine. e.g., The house that is burning is mine.

Lay and Lie
This is the crown jewel of all grammatical errors. “Lay” is a transitive verb. It requires a direct subject and one or more objects. Its present tense is “lay” (e.g., I lay the pencil on the table) and its past tense is “laid” (e.g., Yesterday I laid the pencil on the table). “Lie” is an intransitive verb. It needs no object. Its present tense is “lie” (e.g., The Andes mountains lie between Chile and Argentina) and its past tense is “lay” (e.g., The man lay waiting for an ambulance). The most common mistake occurs when the writer uses the past tense of the transitive “lay” (e.g., I laid on the bed) when he/she actually means the intransitive past tense of “lie” (e.g., I lay on the bed).

See the rest of the list here