When who or that are the subjects of the verb in a relative clause, they cannot be left out.
When who or that are the objects of the verb in the relative clause, you can leave them out.
If, in a relative clause, the relative pronoun is the object of a preposition, it normally goes at the end of the sentence.
What cannot be used instead of that. The meaning of what in the middle of a sentence is the thing(s) that
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