Usage Tip: Who vs. That vs. Which

Which of the following sentences is correct?

a. He's the man that sold me that clunker.
b. He's the man who sold me that clunker.

A simple grammar rule-of-thumb applies here: you use who when you are talking about a person and that when you are talking about an object. Stick with that rule and you'll be safe.

That being said, the rule-of-thumb isn't hard-and-fast. The American Heritage Dictionary says,

It is entirely acceptable to write either the man that wanted to talk to you, or the man who wanted to talk to you. [emphasis added]

Even Chaucer did it, for example.

Now, that vs. which is a bit more complicated -- a more hard-and-fast a grammatical rule. That introduces essential clauses while which introduces nonessential clauses. Some examples:

Essential: I do not trust products that claim "all natural ingredients" because this phrase can mean almost anything. (We would not know which products were being discussed without the that clause.)

Non-Essential: The product claiming "all natural ingredients," which appeared in the Sunday newspaper, is on sale. (The product is already identified. Therefore, which begins a nonessential clause.)