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Contrary to what Microsoft’s grammar checker would have you believe, “which” doesn’t always need to have a comma before it. Whether or not it does depends on what “which” is doing.
With the comma (as in sentence 1), the word “which” is describing the word before it – it’s introducing some extra information that you might find interesting. In sentence 1, all funds are liable for the new tax, and (in case you’re interested) all funds invest at least 80% of their assets. You could take away the words “which invest at least 80% of their assets in UK equities” and the sentence would still be true.
Without the comma (as in sentence 2), the word “which” is defining the word before it – it’s introducing information that you absolutely have to know. In sentence 2, what is at issue is not all funds, but only funds that invest at least 80% of their assets. If you took away the words “which invest at least 80% of their assets in UK equities”, the sentence would no longer be true.
Here are two more examples:
Sentence 1 means “bring me all the library books – and by the way, they’re overdue”.
Sentence 2 means “look through all the library books and bring me only the ones that are overdue”.
In sentences like number 2 (but never number 1), “that” would work just as well – and is often preferable.
Never use “that” to refer to people (e.g. “all the consumers that complained”) – use “who” instead (“all the consumers who complained”). And, of course, avoid using “who” to refer to companies and things.