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In music, we just expect students to open their mouths and sing! Those that can do; those that can't learn very quickly not to be heard.

What is 'those that can do' here, semantically and grammatically?

Semantically, I think it's a sort of specification of the students 'we' expect to open their mouths, but I'm not sure on this one.

Grammatically, I'm at a loss. Is it some sort of ellipsis or something?
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It's a bit of a joke. there is a famous proverb (or could be a quotation, not sure) that says

'Those who can, do; those who can't, teach.'

It means people who are good at something actually do it professionally (i.e. artists) those who are not that good at it teach others (i.e. teachers at art college).
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Nona The BritIt's a bit of a joke. there is a famous proverb (or could be a quotation, not sure) that says

'Those who can, do; those who can't, teach.'

It means people who are good at something actually do it professionally (i.e. artists) those who are not that good at it teach others (i.e. teachers at art college).

I graduated from a teacher-training college where we added to that: those who can't teach, teach others how to teach. Sadly, it was very true in too many instances.
Oh, I see! If there were a comma there in the original text, I wouldn't have missed it.

Thank you, people!

'Those who can, do; those who can't, teach.'

Man...it kind of makes my ears burn...