"Who" is used for people, and "which" is used for things. "That" can replace "who" or "which" (provided there is no comma before).
Find the parrot which ate the stand.
A parrot is a thing - "which" is okay.
Find the parrot that ate the stand.
No comma is required before "which"; therefore, "that" is okay too.
Find the parrot who ate the stand.
"Who" is used for people.
"Who" is used for people, and "which" is used for things. "That" can replace "who" or "which" (provided there is no comma before).
I saw the woman who left it.
person - "who" is okay
I saw the woman that left it.
No comma is required before "who"; therefore, "that" is okay too.
I saw the woman which left it.
"Which" is used for things not people.
The information provided by the "who" clause is not necessary to identify the person; therefore, it should be offset with commas.
Mr Mike Evans, who has sailed in the loch since he was 6, knows it well.
The clause "who has sailed in the loch since he was 6" is not required to identify "Mr Mike Evans"; therefore, it is only additional information and should be offset with commas.
Mr Mike Evans who has sailed in the loch since he was 6 knows it well.
The information provided by the "which" clause is necessary to identify the vase; therefore, it should not be offset with commas.
Why don't you take the vase which you made to the furnace?
The clause "which you made" is required to identify "the vase"; therefore, it should not be offset with commas.
Why don't you take the vase, which you made, to the furnace?
The information provided by the "which" clause is necessary to
identify
the fish; therefore, it should not be offset with commas. "That" can replace "who" or "which" (provided there is no comma before).
...the fish which Lee caught is too small to eat.
The clause "which Lee caught" is required to identify "the fish"; therefore, it should not be offset with commas.
...the fish that Lee caught is too small to eat.
"That" can replace "which" but not ", which".
...the fish, which Lee caught, is too small to eat.
