Grammar is full of gremlins. In fact, someone has even written a book on the subject! I see them all the time in published novels, written by people I admire. I often wonder whether the gremlins crept in on their own (they're sneaky that way), or if they had a little editorial help.
Either way, allow me to introduce you to a few of the gnarlier gremlins and show you how to send them scurrying back underground.
May. Not the month, the possibility. May as a possibility is perfectly permissible when used in the present indicative sense, as in, "I may go to the store this afternoon." However, when I read a sentence like, "You may have been killed!" my immediate response was, "But you can see plainly for yourself, I wasn't killed! I'm right here." What the author should have written was "You might have been killed," which is the proper past tense. The confusion comes when the writer doesn't understand the usage. When in doubt, try substituting can/could in the sentence. "You can have been killed" versus "You could have been killed." That will make it easy for you to see whether you should use may or might.
Who and Whom are both grammar gremlins, depending on how they're used. The easy cure for the who/whom dilemma is to substitute he/him, she/her or they/them. (Providing you're secure in those, of course.) Who is always the subject of the sentence, while whom is the object. Here are a few examples: "Whom did you say is calling?" The subject of the sentence (by the verb usage) is you (did you say). "Who just called?" (The proper usage of who and whom is becoming increasingly rare, which is a pity, because when used properly, they make a sentence much more understandable.
And speaking of he/him, she/her and they/them, most people get in trouble whenever they add themselves or someone else to the sentence. Usually, this is due to having had "---and I" drilled into them whenever they would say something like, "Jimmy and me are going to the store." The authority figure (a parent or teacher) would then say, "Jimmy and I are going to the store." The unconscious response to this is to believe that "and me" is never correct. Nothing could be further from the truth. "The candidates for Writer of the Year are between Joe, Jenny and I." (cringe) If that is the case, depending on who is in charge of the award, you won't be getting it.
Past tense is another place where writers can get in trouble. Because of the way certain words are pronounced, with the endings swallowed, it's easy to forget the spelling differences required. Compare "I use to go to the store on Thursday." to "I used to go to the store on Thursday." In much of the United States, at least, the two sentences sound identical. In print, however, use the present tense, and you will look ignorant.
It isn't just word usage in past tense that can be a problem. Too many times lately, authors of recent books don't make multiple verbs agree. E.g.,
"I had got up early and went to the park." Eeeeeeek! No! It should say, "I had got up early and gone to the park." If you use past perfect for the first part of the sentence, you must use it for the second part, unless the two clauses are separated by a comma. That could sufficiently separate the two events in time. However, it's still better to make the verbs agree with each other. It keeps you from looking like you slept through English class.
Another disturbing trend in past tense is forgetting all about past imperfect, and using past perfect without the auxiliary verb. Frankly, I blame whoever wrote "Honey, I Shrunk the Kids." Every well-educated person who first heard about that charming Disney film shrieked in horror. It should either have been "Honey, I Shrank the Kids," or "Honey, I Have Shrunk the Kids." (Well, you know how well that went over.)
Show you're well-educated in your written language of choice. Use the right grammar and the correct words. You'll make fans whom you may otherwise lose.