How do you know when to use "who" or "whom" in a sentence? 

 This is one of those tricky grammar questions that continues to confound some people. According to the American Heritage Book of English Usage, "Who is used for a grammatical subject, where a nominative pronoun such as I or he would be appropriate, and whom is used as the object of a verb or preposition."
 
For those of us to whom (notice the correct usage of "whom" after a preposition) grammar school is a distant memory, a little basic English review is needed. 
A nominative pronoun acts as a subject of a verb (Who ate the cake?) or as the subject of a linked verb (Did you see who ate the cake?). A direct object, on the other hand, is the object of a verb (Whom did you call?) or a preposition (He is the person to whom I placed the call).
 
If the days of diagramming sentences on the blackboard are fuzzy, and subjects and objects swim together in a sea of confusion for you, many sites advise you to simply trust your ear. 
The Blue Book of Grammar and Punctuation offers this helpful tip: if you can replace a word with "he" or "she," then it is the subject of the sentence and you should use "who." If you can replace the word
 with "him" or "her," it is the object and you should use "whom."
You might need to rephrase the sentence to make this work. 
On the web site of the Meredith College Writing Center, we found another handy rule of thumb -- only pronouns that are objects end in the letter "m" (whom, him, them). 
The correct usage of these troublesome pronouns is often ignored in speech and informal writing when the word "whom" would sound forced or unnatural.