whom

Whom is the objective form of who.

You should always use it after a preposition, where an objective form is expected: With whom did you attend the reception? The package doesn’t show for whom it’s intended.

When whom is removed from the preposition that governs it, technically you should still use whom, although today speakers and writers are less particular about this than in the past and many people find it acceptable to say, e.g., Who did you attend the reception with and The package doesn’t show who it’s intended for.

When who is the object of a verb, many speakers today do not use whom: They said there would be an arrest but they didn’t say who.