Relative pronouns connect clauses and avoid repetition. Learn when to use who, whom, whose, which, that, and the relative adverbs where, when, why.
That is the factory in ...... my Dad used to work.
The woman who said hello was my old dance teacher. Although it can be used to refer to a person, 'that' is not correct as the subject of a clause (i.e. that said hello). You will hear 'that' used incorrectly in informal spoken English, however