The short answer: Use which when the clause adds extra information and that when the clause is essential to the sentence.
People search this because they want a fast memory trick, not a grammar lecture. The easiest way to remember it is to ask whether the information can be removed.
Quick Notes
- If the sentence still makes sense without the clause, which is often the better fit.
- If the clause is required to identify the thing, that is usually the better fit.
- Commas are a clue that the clause may be extra information.
Examples
- The shirt that I bought is blue.
- The shirt, which I bought yesterday, is blue.
Writers often use this rule more than students realize because it makes sentences cleaner and easier to read.
