pronoun
1
BAD : A woman can do everything what a man can do.
GOOD : A woman can do everything (that) a man can do.
BAD : I'll do all what I can to help you.
GOOD : I'll do all (that) I can to help you.
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What is not used as a relative pronoun. After all, everything, anything etc, use that or nothing: 'You can have anything (that) you like.' 'I have everything (that) I need for the time being.'
2
BAD : She told him that she didn't want to marry him, what in my opinion was very silly of her.
GOOD : She told him that she didn't want to marry him, which in my opinion was very silly of her.
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What is not used as a relative pronoun. When you comment on a previous statement, use which : 'Lizzie ate the whole box of chocolates, which was very greedy.'
3
BAD : Please tell me what would you like for a wedding present.
GOOD : Please tell me what you would like for a wedding present.
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When a wh- clause is part of a sentence (e.g. the subject or the object), the subject and verb in the wh- clause do not change places.
Compare: 'Why did she leave so soon?' 'Do you know why she left so soon?'