Back in the days of Ronald Reagan, Republicans were arguing about whether to hold the line on taxes. Moderate republicans wanted to make the following proposal: “We oppose any attempts to increase taxes which would harm the recovery.”
Conservative republicans made a small change to the proposal. They inserted a comma before which, to read: “We oppose any attempts to increase taxes, which would harm the recovery.” The revision was passed, but of course it entirely changed the meaning of the sentence. The new proposal ruled out all tax increases.
Grammatically, the conservative republicans were right to insert the comma before which. People have a lot of difficulty knowing how to use that or which. Here’s the rule:
That is used to introduce a phrase that is essential to the meaning of the sentence, such as: He wants to buy the car that has only 5,000 miles on it. A that phrase is usually not preceded by a comma.
Which is used to introduce a phrase that is not essential to the meaning of the sentence, such as: He wants to buy the red car, which has leather upholstery and a sun roof. A which phrase is usually preceded by a comma.
It may seem like a small difference, but it can have a huge impact on the meaning of a sentence. If moderate republicans had used that instead of which, who knows how history might have changed. Perhaps the very rich would be paying more taxes!