Who, Whose, Whom and Who's

Whose and who's.

Simple. Who's means who is. Always. Just like it's always means it is. No exceptions.
You can't say "Who is book is this?" can you? So here it must be whose. You can't get much easier than that.

Now the more difficult one: whom and who.
Well, in fact this one is also pretty easy, but there is one problem - whether we should use whom at all.

Now I'm f

rom the olden times, back in the days when school wasn't expected to be fun and teachers happily taught forty minute classes of pure GRAMMAR. I actually say things like, "Whom did he give it to?" Yeah, I know. People like me are anachronistic leftovers from a bygone era. I admit it.

Anyway, let's look at the basis for the difference first.

• Who is the subject of a verb, like he or she. Subjects do things.
• Whom is the object, like him or her. Objects have things happen to them.

• Who is that? [Who is the subject. ]
• Whom did she see? She saw whom? [Here, "she" is the subject. Compare: Did she see him?

"She likes him." should become in a question "She likes whom?" or "Whom does she like?".

The man, whom they all knew to be a doctor, came running into the room. [In this sentence, the subject of the main sentence {in red} is "the man". Ignore that part of the sentence and look at the other part. The subject of the blue bit is "they". They all knew him to be a doctor. - So you can't use "who" in this part of the sentence.

Compare that last sentence to this one:

The man, who was a doctor, came running into the room.
In this sentence, the man is still the subject of the main [red] part.
He's also the subject of the blue part. He's a doctor. This time, there's no other subject like we to worry about. He was a doctor. Which is why we use who and not whom.

We use whom after prepositions too: by whom, with whom, to whom etc. Always. At least always if you want to be grammatical...*grin*.
The people with whom I travelled were all from Nannup.

Another problem with the preposition + whom is that it so often ends up with a hanging preposition which is just plain ugly. Look at this: He didn't know whom to give it to. And yet He didn't know to whom to give it sounds stilted.

Ok so now you know: you can't say "Who did you give it to?" [in other words, "To who did you give it?"] Bad grammar. And hands up everyone who's going to obey that grammar rule...?
Which brings us to the real problem. Whom has gone out of fashion. Put it in your writing and you can sound really staid and out of date. On the other hand, if you use who when you should use whom, it is going to grate on old pedants [one of whom may be the editor you are trying to impress] like me. So what's a poor writer to do?

Well, if you are writing a modern novel, I would not use whom in your dialogue [unless someone from a past age like me is speaking!]. If you are writing a period piece on the other hand, and your speaker is a well-bred lady/gentleman, then perhaps you should.

And in your text? Tough one. Theoretically you should be grammatically correct. But...you don't want your book to sound like a nineteenth century tome. So dodge whom altogether whenever using it just doesn't ring true to your writer's ear. In cases like that, rewrite the sentence to avoid it. I know I do.