Determiner asking for a choice among known options before a noun

• Used to specify one or some from a set

Pronoun asking which one or ones among known options

• Stands alone without a following noun

Relative pronoun introducing a nonrestrictive clause about a prior noun

• Can refer to a whole previous statement or situation

Relative pronoun or determiner introducing a restrictive clause modifying a noun

• Often interchangeable with that in modern use