[Grammar] We will do what we can to help.

[kadioguy]

In the OALD9, it says:

do
verb

used to refer to actions that you do not mention by name or do not know about

We will do what we can to help.
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Why not We will do what we can do to help?

Why is no main verb in what we can?

 

[GoesStation]

The second do is implied. Can can work this way.

 

[Matthew Wai]

Can other modal verbs work this way too?
'I must do what I should.'

 

[GoesStation]

Can other modal verbs work this way too?
'I must do what I should.'

Yes.