Use of a Dash

John SongCan a dash link two clauses as in the sentence below?Tom couldn't come to his best friend's wedding--it was a shame.

Yes. The m-dash is an informal colon, parenthesis, comma and other punctuation.

John SongSome say "it" should be replaced by "which".

That's a good idea.

What is a good idea? Do you mean replacing "it" with "which"?
Actually, after talking to English composition experts, I was convinced that "it" would work better than "which".
For I would probably say "which", but in writing I think I should opt for "it". Thanks.

John Songafter talking to English composition experts, I was convinced that "it" would work better than "which".

I doubt their credentials, then.

In my experience, most people who are good at grammar and English composition avoid dashes.

Clive

Clive most people who are good at grammar and English composition avoid dashes.

Not I, however. I find them extremely useful and quite effective in some writing.

But you said an em dash can replace a colon in informal writing.
That sounds contradictory to your argument that "which" is a correct choice, not "it".
If the dash in the sentence can replace a colon, I think "it", not "which", would be the correct choice.
I'm more confused now.

John SongThat sounds contradictory to your argument that "which" is a correct choice, not "it".If the dash in the sentence can replace a colon, I think "it", not "which", would be the correct choice.

Change of punctuation can involve other choices as well. I would use one of these myself:

Tom couldn't come to his best friend's wedding, which wa

I think this was one of the questions one university in Korea put on the English test.
I think they should have really thought through before making this kind of "controversial" question for an entrance exam.
Thanks for you input anyway.

Hello John Song.

You don't know how they treated the replies.

You write a little as though the university thought that there was only one correct answer, which may not have been the case.

Maybe you have insider knowledge, but, without it, I'm inclined to give them the benefit of the the doubt. Maybe I'm too generous.