If there’s any grammar bugaboo that confounds people, it’s when to use “who” and when to use “whom” (it’s right up there with the lie/lay and effect/affect conundrums).
First, stop worrying – in the pantheon of things to worry about in the world, “who” and “whom” barely crack the top 10.
Second, when it comes to conversational English, you can almost always drop “whom” altogether and rely on “who” – except, perhaps, if you find yourself sipping sherry with the Queen of England (understandably, she’s a stickler about these things) or reminiscing with your 7th grade language arts teacher.
In formal or semi-formal written English, however, understanding when to use which word is still handy (that said, when in doubt, always default to “who”; there’s nothing more stuffy sounding than a mistaken “whom,” while few but the most persnickety harrumph over an indecorous “who”).
I know what you’re thinking now: nice in theory, Dan, but spare me the complicated grammar lingo. Fair enough: Here are two rules, in descending order of commitment.
Rule # 1: Substitute “he” for “who” and “him” for “whom.”
Rule # 2: If you have to think about this for more than 10 seconds when writing, choose “who” and move on. Life’s short enough as it is.
Note: Another "who" dilemma is between “who” and “that.” A person can be either a “who” or a “that” but a thing is always a “that.” As far as dogs, cats, platypi, and the rest of the animal kingdom go, it’s a finer distinction. If you’re referring to random animals, they’re “that,” but if you’re referring to animals whose names you know, they’d be “who.” Dogs that have long hair are my favorite animals. Not surprisingly, my dog, Bailey (below, yawning at the ho-humness of it all), is a dog who has long hair.