TO or not to use TO in this question: What she likes to do is (to) cause problem.

[Englishnow]

All what you do is (to) cause problem.

What she likes to do is (to) cause problem.

Is it wrong or optional ? Is there any rules about this situation ?
Thanks very much.

[Alan]

Hi,

I would prefer:

All that you do is cause problems/a problem.

What she likes to do is cause problems/a problem.

In both sentences 'to' isn't needed.

Alan
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English as a Second Language
You can read my ESL story Present Simple