That, Those, Who, Which

 'That' is used as a determiner at the beginning of sentences to indicate one object which is far from the speaker. Note that the plural form of 'that' as a determiner is 'those.' 'That' and 'those' is generally used with 'there' to indicate that the object(s) is not close to the speaker. In this case sign  this and that or sign question this and that.

That used as a relative pronoun is not signed.

That' can be used as a relative pronoun to connect two clauses. In this case, 'that' can also be substituted by 'who' or 'which.'

Examples: That = Which

Tom bought the apples that the man was selling.
OR
Tom bought the apples which the man was selling.

Examples: That = Who

Peter invited the boy that was new in class.
OR
Peter invited the boy who was new in class.