Use relative pronoun: which (or who) and that

[Splove82]

I wonder about relative pronoun.

sometimes do i have to write "that" instead of "which"?
or sometimes do i have to write "which" instead of "that"?

ex) It was Gremio who looked nervous.

In this sentence, can I change "who" to "that"? if not, can you explain why?

also, can i add comma after Gremio? if not, why?

Please explain in details~~

Because I wrote that as a relative pronoun in a sentence, but my teacher said to me you should change "that" to "which". I don't know why~~

[Mister Micawber]

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Start reading HERE.
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Native English teacher at Mister Micawber's

[Torsten]

Hi Splove,

You also might want to study this relative pronoun lesson
http://www.english- test.net/lessons/21/index.html lesson by Alan Townend.
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TOEIC Preparation & TOEIC Vocabulary

[Prezbucky]

I'd go with who or whom (as opposed to that) when referring to people (differing slightly from MM's site, which deems both who/whom and that to be acceptable).

Not that that is terrible. I've simply pitched my tent in the who/whom camp on this issue. hehe
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[Yankee]

However, the word 'whom' would always be wrong in splove82's particular sentence.

[Lost_Soul]

[Yankee]