I want to add something to the discussion of that/which/who(m) clauses.
Schoolmarm has given a succinct explanation of essential and non-essential clauses (though I also have doubts about that essential clause example.
) And the rule she gave for using that and which is the rule taught in US schools today, though I'm not certain whether it's a hard-and-fast rule or merely a de facto norm in the US as a whole.
However, the older rule, which is still in use in Britain and other countries, is that "that" may only be used to introduce an essential clause, but "which" may be used to introduce either type. The commas, not the pronoun, determine which type of clause it is. Sometimes punctuation can be the only difference.
The mare, which won the Grand National in 2000, had a promising foal last year. [non-essential clause; evidently the reader knows from context which mare is meant]
The mare which won the Grand National in 2000 had a promising foal last year. [essential clause; the reader wouldn't know which mare is meant without it]
"That" can replace "which" only in the second sentence.
So, for GEs editing fics written in UK English, you should leave the second type of "which" alone.
