pronouns - That vs Which when talking about a subject

This is actually the title of a question I just asked a minute ago ::- ). While writing it, I remembered one of my most terrible dilemmas ever: when to use "which" and when to use "that"? Even worse, Microsoft Word sometimes forces me to add comas before "which".

Phrase in question:

What tense to use when writing about an ongoing action in a text which will be published / read in the future

vs

What tense to use when writing about an ongoing action in a text that will be published / read in the future

Answer

which and that are both correct.

I don't think you really need our help on this. You just need to have more confidence in your own writing. But if you really want to know:

The cat, which was sitting in the tree, miaowed.

Here, the sub-clause sitting in the tree gives you extra information. The commas around which was sitting in the tree show that you could delete the sub-clause, and the sentence would still be valid:

The cat miaowed.

For this kind of sub-clause, which we call non-restrictive, you should use which.

Now compare to:

The cat which was sitting in the tree miaowed.

The cat that was sitting in the tree miaowed.

Here, the sub-clause specifies which cat you are talking about. If you delete the sub-clause the sentence is no longer valid. It is called a restrictive sub-clause.

In a restrictive sub-clause, that and which are both fine. Some of the more prescriptive style guides recommend that you should reserve which for restrictive sub-clauses, but that is really just a matter of taste.

MS Word's style checker follows these more prescriptive style guides, so when it sees a which it assumes that you are using a non-restrictive sub-clause, and insists that you add commas. Needless to say, MS Word is wrong.