We learned in this article what "whom" means, but when exactly are you supposed to use the word "whom?" Well, all you really have to do is determine whether the verb in the sentence is performed by the word or on the word.
One would not say, "Zachary spoke to he." Instead, they would say, "Zachary spoke to him." Because the action of speaking is performed on "him," the objective case is necessary. In the same manner, traditional grammar dictates that one should ask, "To whom did Zachary speak?" rather than "Who did Zachary speak to?"
It should be noted that native speakers of the English language often do not use "whom" in such situations, merely because of the changing nature of language. "Whom" has become less common in recent centuries.
If looking for the object of the verb is too difficult, just remember this. If you can replace the word with "him," "her," or "them," then use "whom," but if you can replace the word with "he," "she," or "they," then use "who." Following this simple rule will help you to improve both your writing and your speech!