That vs. which

My husband, Brady, took me fishing this weekend, which is an activity I find very boring. So I pondered grammar to pass the time—that is until Brady caught a fish that died as he was trying to release it back into the lake. As that poor, doomed bass floated by, my thoughts turned to the beauty of life, the cruel inevitability of death, and then, naturally, to the difference between that and which.
 
Wait… there’s a difference?

• Yes! “That” and “which” cannot be used interchangeably, for “that” denotes a restrictive clause and “which” denotes a nonrestrictive clause.

Say whaaaaaat?

• A restrictive clause is a part of the sentence you cannot get rid of without changing the meaning
  • The fish that died haunted me all night.
    • Removing “that died” changes the meaning of the sentence, as “The [presumably alive] fish haunted me all night” doesn’t make as much sense.
• A nonrestrictive clause is extra fat: getting rid of it will not change the meaning.
  • The fish died, which haunted me all night.
    • You can remove “which haunted me all night,” and the meat of the sentence is not really affected.

So you could say “that” helps define a sentence, while “which” offers extra information or helps clarify the meaning of the sentence as a whole?

• Exactly.
  • Fishing is a sport that Brady loves.
    • In this sentence, “that Brady loves” reflects on sport and helps define it in terms of his preferences.
  • Fishing is a sport, which Brady loves.
    • This sentence is looser, as it implies that Brady loves the fact that fishing is a sport rather than the sport of fishing itself.

What happens if I mix them up?

• Catastrophes upon catastrophes! Consider the following example:
  • My husband killed the fish that was sad.
    • This implies that the fish was sad before he died, and since I was there I can tell you the fish was quite content before his demise. Therefore, this sentence is not true.
  • My husband killed the fish, which was sad.
    • This implies that the act, as a whole, was sad. And it was.

How do I punctuate “that” and “which”?

• “That” requires no additional punctuation. (Think: “That was easy.”)
• A “which” clause, which is a little harder to remember, is set off by commas.

I hope this clears up some grammatical issues that have been haunting you.