It's the same as the explanation for any use of the emphatic mode:

"I do catch" vs. "I catch". The purpose of the do/did is emphasis.

I can't imagine any case where "who did catch whom?" would be used.

It is a so called subject question. In this case the English language has no "circumscription" with a form of to do:

- Who has seen my book?
- What surprised you?
- Which boy has brought the letter?

Yes I understand that, Eric. The question was why it isn't used. We say for example: Whom did Mary see? Why, then, don't we say: Who did see whom?

you can certainly say "who did see whom?"

Example:

First I though Dave was dating Joanie, but then it seemed like Joanie was hanging out more with Jim, and Dave and Katie got along great, too. The whole thing was very confusing at the end.

So who did see / was seeing whom then, do you think?

So you don't want the explanation you give to help students remember when to use "do"; you want the historical explanation of how "do" came to be used in English? That's harder; AFAIK none of us are experts in the subject.

I just want a simple explanation why we'd rather say "Who saw whom" instead of "Who did see whom". An explanation that students will understand. Reinhard W. already touched on it but perhaps somebody could elaborate?

Thanks for all your help.

Certainly you can, dude, but that's for emphasis, wouldn't you agree?

As far as a straightforward question, with no particular emphasis, is concerned, Reinhard is correct: It's the difference between a subject question and an object question surely.

Who caught whom? Subject question.
Whom did s/he catch? Object question.

Ok, then.

You write "do" in a question before rhe subject:

Mary likes fish.
What does Mary like?

After "do" the next thing is always the subject.
When the question IS the subject, obviously you can't put the subject after "do", so there is no "do".

Mary likes fish.
Who likes fish?
Not: Who does Mary like fish?

yes, it is for emphasis. I didn't mean to imply anything else.

Great, that's exactly what I was after. Thanks a bunch:)

"Subject Questions
There are also subject questions. These are questions that we ask to find out about the subject. When what, which, who or whose refers to the subject, the question word comes before the verb without the use of the auxiliary verb.

For example:-

If the answer is "The train to London was late." the question would be "Which train was late?"
If the answer is "I won the race." the question would be "Who won the race?"

More examples:-

Object questions:-
What did you do today?
Which film did you like best?
Who did I phone?

Subject questions:-
What happened today?
Which film is best?
Who phoned me?

http://www.learnenglish.de/grammar/questionte...

Claudia came first. :-)

Nein, eigentlich du in #2. :-)

Note that it is indeed possible to ask:

1. Whom did John catch?
2. Whom did who catch?