You can always tell a grammar perfectionist by their correct use of "who" vs. "whom". The rest of us often just hope we get it right when we choose between the two pronouns. However, there's an easier way to resolve the "who vs. whom" problem than using one or the other and hoping you get it right.
Step 1: Know the difference between subject and object in a sentence
The first step to knowing whether to use "who" or "whom" is understanding the difference between the subject and object of the sentence. When a sentence is written properly, it should have a subject and a predicate.
A subject is the person or thing that is being discussed or described. A predicate is the part of a sentence or clause that expresses what is said of the subject and that usually consists of a verb with or without objects, complements, or adverbial modifiers.
For example, in the following sentence, "Sheila" is the subject and "gives" is part of the predicate.
Example: Sheila gives her kids hugs every day.
Step 2: Find the object of the predicate
Now that you understand the difference between the subject and predicate, let's look at how to find the object within the predicate. As shown above, the predicate of a sentence is where the action takes place and is the part of a sentence that usually consists of a verb with or without objects, complements, or adverbial modifiers. So now, the next step in determining whether to use "who" or "whom" is to figure out if either of those pronouns replaces a noun that is an object in the sentence.
So let's look at the definition of an object of a sentence. The object is the person or thing receiving the action of the verb, or to whom the actions are being done.
Consider this sentence: She invited Drake to perform at her party.
The subject of the sentence (or the person doing the action) is "she". The object of the sentence (or the person receiving the action) is "Drake".
Step 3: Replace the subject with "who" and the object with "whom"
Now, you've reached the final step in determining which of these often confused pronouns to use. All that's left to do is replace the subject with "who" and the object with "whom".
So, who invited whom to the party?
She invited Drake to the party.
(Another pro tip: If the question can be answered with "she" or "he", use "who"; if a question can be answered with "him" or "her", use "whom".)