Do you lose sleep over rules of grammar?
I usually don't, because I was lucky enough to grow up in a household where both parents insisted that I speak correctly all the time. My father was a writer and, frankly, a snob about English. His mother (who died before I was born) was from Oxford, and he spoke with an English accent his entire life, even though he'd never stepped off North American shores.
We were required to pronounce the word tomato, toe-MAW-toe, or get in trouble. Trust me, there were no ain'ts in our house, no sentences without verbs, and no double negatives.
As a result, I can usually count on my ear to guide me, even when I don't fully understand the grammar rule.
The one exception? Who versus whom. I always have to think really hard about which word to use, even though the grammar isn't terribly complicated.
I think my hesitation arises because so many people don't bother using "whom" at all; they just say "who." As a result, my ear (and likely yours) has never been exposed to the necessary "training" to make the correct choice.
So, here is the rule: Who is a subject, and whom is an object. If I yell at my son (because he just ate the last cupcake), then I am the subject and my son is the object. The subject of any sentence is the person doing something, and the object is having something done to them.
You use "who" when you are referring to the subject and "whom" when you are referring to the object. How do you figure that out, you ask? Well, the quick trick is to answer the question with either "he" or "him":
Take: Who/whom were you yelling at? And ask yourself which of the following makes more sense:
I was yelling at him.
I was yelling at he.
Clearly the first choice makes more sense. Him. And because "him" ends with an M, you need to use whom (which also ends with an M.)
Let's try another example: Who/whom won the Pulitzer Prize for fiction in 2012? Now ask yourself, would you say:
S/he won the Pulitzer Prize for fiction in 2012.
Her/him won the Pulitzer Prize for fiction in 2012.
Clearly, the first choice makes more sense, so "who" is the correct word.
Actually, that was a trick question. No one won the Pulitzer for fiction in 2012. The judges declined to award a prize that year.