The simplest way of distinguishing between who and that is to think of
- who as referring to people
- that as referring to things.
For example:
She’s the woman who invented the new medicine.
The house that needed repainting was destroyed in the fire.
However, some grammar experts think it’s okay to use that for persons generally and who for particular persons. For example:
Tradespeople that don’t turn up on time are infuriating.
The tradesperson, who turned up hours late, couldn’t fix the problem.
I think it’s simpler to stick to who for people and that for things.
Tradespeople who don’t turn up on time are infuriating.
Online grammar programs
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