The distinction between 'who' and 'whom' can feel like navigating a maze at first, but it becomes clearer when you break it down. 'Who' serves as the subject of a clause, while 'whom' is the object. A trick I've found helpful is rewriting the sentence with 'he' or 'him'—if 'he' fits, use 'who'; if 'him' fits, 'whom' is correct. For example, 'Who/Whom wrote the letter?' becomes 'He wrote the letter,' so 'who' is right.
Practicing with examples from literature or dialogue in 'Harry Potter' can make this feel less abstract. Spotting the difference in well-written texts trains your ear over time. It’s also worth noting that in casual speech, 'whom' is often replaced by 'who,' but knowing the rule adds polish to writing. The key is to not overthink it—let patterns emerge naturally through exposure.