What

What = used with noun to ask question seeking information about something or somebody: ‘What is your name?’
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.’
When you comment on a previous statement, use which: ‘Lizzie ate the whole box of chocolates, which was sent by her uncle.’
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: ‘Why did she leave so soon?’ ‘Do you know why she left so soon?’