Once you know the difference between whom and who, it’s easy to keep them separate. As common with pronouns, who has two distinct forms, the subject: who, and the object: whom. So, much like he vs. him, who is always the subject of a sentence and whom is always the object. For example, “Who gave the ball to whom?” is a correct sentence, as “who” is the subject and “whom” is the object. However, the sentence “Whom do you know in Denver?” is also correct, because even though “whom” appears at the beginning of the sentence, it is the object, with “you” being the subject.
The confusion of lay (a transitive verb meaning “to put” or “to place”) and lie (an intransitive verb that means “to recline”) often stems from the fact that lie’s past tense has the same spelling as lay’s present tense: both are spelled “lay.” For many, lie vs lay grammar is tricky, but the easiest way to tell when one word should be used over another is if the verb is directly followed by a direct object. “She lay the hat down” uses the verb lay, because the direct object, “the hat,” directly follows the verb. “That cat lay on the bed,” however, uses the verb lie in its past tense, as “on the bed” is a prepositional phrase, not an object.
Many people view these differences as too subtle to learn. However, people wanting to sound professional in both their speech and writing would do well to observe the obvious differentiations.