Trick
You can try out the simple substitution he/him test. Insert the word 'he' or 'him' into your sentence. Decide which one sounds appropriate. If 'he' sounds best, use 'who'. If 'him' sounds best, use 'whom'. Try it out!
(Who or whom) ate the last banana?
'He ate the last banana?' -Or- 'Him ate the last banana?' Which one sounds best? 'He ate the last banana' sounds best. The answer must be who! Who ate the last banana?
(Who or whom) did you invite to your birthday party?
'Did you invite he to your birthday party?' -Or- 'Did you invite him to your birthday party?' Which one sounds correct? Him sounds best! So the correct answer would be whom. Whom did you invite to your birthday party?
With (who or whom) are you going to the park?
'Are you going to the park with he?' -Or- 'Are you going to the park with him?' Which one is best? The use of him sounds most appropriate. 'With whom are you going to the park?' is correct.
More Examples
Here is what we have learned so far:
- Who is always the subject. When using the simple substitution, who = he.
- Whom is always an object. When using the simple substitution, whom = him. (Notice that whom and him both end with 'm.' Just another little trick to help you)
(Who or Whom) went to the store?
The subject went to the store. 'He' went to the store. So 'who' is the correct option. Who went to the store?
(Who or Whom) does Alex like best?
Don't be fooled just because 'Who' or 'Whom' is the first word in the sentence. This sentence is about Alex - Alex is the subject and is performing the action of 'liking.' So, we are looking for the object in this sentence, and we have learned that whom = object.
Reword the sentence: Alex likes (who or whom) best? 'Alex likes 'he' best?' -Or- 'Alex likes 'him' best?' 'Him' is the correct answer. So, 'Whom does Alex like best?' is correct.