That Vs. Which

That Vs. Which

This is a one-way error: Which often gets used when that is needed. That is rarely, if ever, used mistakenly in place of that.

Which requires a comma before it. The phrase starting with which provides additional information that is nice to know but is not essential to know.

That will never have a comma before it and begins a phrase with information that you need to know.

Often the comma will be enough of a clue for you to know which to use.

I went to the parking lot to my car, which is a red van.
I went to get my car that is a blue sedan.

In the first phrase, which is a red van is a statement that is “nice to know.” I have one car, so it’s already clear that it’s a van. In the second phrase, I have more than one car. The phrase that is a blue sedan is “need to know” information. You don’t know which car I mean without that clarification.

Here are two different ways to say the same thing using which and that.
The snow was falling heavily, which made it hard to see.
The snow that was falling heavily made it hard to see.

Wrong examples (from the book I’m reading):
He was never as happy as when he had a job which required no thinking at all.
If you only do the easy and useless jobs, you’ll never have to worry about the important ones which are so difficult.

Summary

That = Need to Know = No Comma

Which = Nice to Know = Comma