When to use who:

• When referring to the subject of the sentence (or in a section of the sentence or clause)
  
• When who can be replaced with he logically
  
• When who has a verb to accompany
  
• When who can be replaced by that

Examples:

#1.Who went to the pub yesterday?

#2.He was the one who stole it, officer.


In the examples above, we can see that the instances of who could be replaced by he (Example #2 could be seperated into two independant clauses: He was the one. He stole it, officer.). These examples also show that who had a verb to accompany it (in other words, who will do something in the sentence). In Example #2, the who could be replaced by that without changing the sentence's meaning. Both instances of who were the subjects of the sentences (or, in Example #2, the second independant clause). The instances of he could have been replaced with I as well.

Now this is a nice word. - Educated Snoipah


When to use whom:

• When whom refers to the object (or indirect object) of the sentence or independant clause
  
• When you place whom after prepositions
  
• When whom can be replaced with him logically
  
• When there is no verb to accompany whom

Whom isn't just in the English language to sound fancy; in fact, it can clarify a sentence and protect you from writing a dangling meaning (when the sentence could be interpreted in more ways than one).

Examples:

#1.Whom shall I fear?

#2. Sheldon went with whom to the party?

#3. He saw whom yesterday?

#4. Whom are you talking to?

In these examples listed above, the sentences could be rearranged to form a declarative answer to the questions. They also all are able to replace him.

Examples:

#2. Sheldon went with him to the party.

#4. I am talking to him.


Stated above, him is the quick way of finding out if it's whom or who for referring to nouns in a sentence or clause.

In Example #1, I is the subject; fear is the verb that accompanies the subject (shall is an auxiliary verb). Since there is no verb after whom in this sentence (it has no verb to accompany), whom is the direct object (the object to which the action is happening to). Rearranged, this sentence would read this way to be declarative: I shall fear him/you.

Rearranged, all of the sentences with prepositions had whom after the preposition.

The declarative forms of these sentences could have had him replaced with you (I believe).

- John Cleese

Thanks to a certain Falling Melon, I've taken the liberty of explaining just why whom exists. From what I can surmise (as not many people have it etched in stone), whom was created to better understand just which noun was being referenced in the sentence or clause.

I hope you enjoyed my guide! Thank you for your time.

Note: if it helps, you can imply a preposition in front of a possible object for clarifying.

Note: Steam still doesn't know what a paragraph is!

In case you didn't know, your is possesive; you're is a contraction of you are (again, I understand that some may have Dyslexia or are simply using "LOL Language," but you're not somehow magically offended by me just pointing this out, are you?).