“That” is a restrictive pronoun. It helps you specify something about the noun:
I’m going to wear a white shirt that I bought last week.
You know this isn’t just any white shirt, although if this is the only sentence you have, it doesn’t tell you much about it. But at least you know that I need to tell you about last week’s shopping trip. The phrase “that I bought last week” is sometimes called a content clause.
“Which” introduces a relative clause. A relative clause tells us something about the noun. This something may not be an essential piece of information:
We need to wear a white shirt, which you can buy in any High Street.
This is a statement of more general information.
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