That vs which.

I had never even thought there was a difference between that and which until we were taught in one of my grammar classes in college. At first I couldn't understand when to use which. That with a restrictive clause and which with nonrestrictive? What did that even mean? Nonrestrictive which uses (what seems to me like) restrictive commas, but don't use commas with that. I couldn't get it straight in my head.

At some point it clicked, and now it's like second nature. Let's break it down. It will make more sense when we apply it to the examples, so try to get through the next paragraph.

That and which are both relative pronouns, meaning they are used to connect an additional thought or clause to the main thought. So they introduce a relative clause that relates to the main clause. They each have a specific function. That is used to add restrictive information to the main clause; it specifies or narrows the main clause with necessary information. Which is used to add nonrestrictive information to the main clause; instead of narrowing the main clause, it adds description to the main clause. This additional information is preceded by a comma.

Examples:

They would catch any Yankees that might try to pass through. 
Using that  in this sentence means that they are only going to catch the Yankees that try to pass through. They are not going to catch all Yankees, some of which might pass through. 

It stopped at his eyebrows, which protruded like thick, hairy awnings.
Perhaps the trickiest part with that and which is that they can be interchanged depending on what the author meant to say. Here, however, we have "eyebrows" as the item being described by the following relative clause. Because the relative clause is just additional description and could be deleted without removing the important part of the sentence, comma which would be the appropriate choice.


Clint sat on the ground, his back to a tree that seemed all trunk and no limbs. 
This one could probably be either that or which, although I like that more because I think the description of the tree adds a lot to the sentence and the scene of Clint resting against the tree. However, this could also be argued about the previous sentence. In each case, I left it how the author had written it, although I did add a comma to the previous sentence to make the which clause grammatically sound.

The trees moved with the wind, which now came from their backs.  
The author hadn't mentioned the wind before, so I felt that the additional information about it coming from their  backs was nonrestrictive; it was just extra information. Cue comma which.

The wrap-up: Use that to connect essential description to the main clause without a comma; use which with a comma to connect additional, nonessential information to the main clause. Think about what the sentence would meaning using both of them; if you're not sure what an author meant, query!