That, who or which?
‘That’ defines a thing (not a person). ‘Who’ defines a person (not a thing). ‘Which’ gives extra information, often in a clause with commas round it.
Who or that?
Miranda, who used to live in Melbourne, now works in London.
Melbourne is a city that’s known for its coffee culture.
That or which?
This is the project that Miranda managed.
This project, which Miranda managed, is a roaring success.
As a rule of thumb, if you’ve got commas in there, go with ‘which’.
What about ‘whom’?
It’s on its way out. So if you’re not sure you’ve got it right, leave it out.
But the rule is: if you’re talking about a person, and in the next bit of the sentence that person is the object (so, the verb is being done to them), then you use whom.
Miranda, whom we ferried to the pub, bought the first round.
Miranda, who doesn’t like cheese, will still come for pizza sometimes.