If the sentence doesn’t need the clause that the word in question is connecting, use which. If it does, use that. (Pretty easy to remember, isn’t it?) Let me explain with a couple of examples.
- Our office, which has two lunchrooms, is located in Cincinnati.
- Our office that has two lunchrooms is located in Cincinnati.
These sentences are not the same. The first sentence tells us that you have just one office, and it’s located in Cincinnati. The clause which has two lunchrooms gives us additional information, but it doesn’t change the meaning of the sentence. Remove the clause and the location of our one office would still be clear: Our office is located in Cincinnati.
The second sentence suggests that we have multiple offices, but the office with two lunchrooms is located in Cincinnati. The phrase that has two lunchrooms is known as a restrictive clause because another part of the sentence (our office) depends on it. You can’t remove that clause without changing the meaning of the sentence.
- The time machine, which looked like a telephone booth, concerned Bill and Ted.
- The time machine that looked like a telephone booth concerned Bill and Ted.
In the first sentence (thanks to the use of which), the time machine concerned Bill and Ted. It also happened to look like a telephone booth. In the second sentence (which uses the restrictive clause), Bill and Ted are concerned with the time machine that looks like a telephone booth. They aren’t concerned with the one that looks like a garden shed or the one that looks like a trailer.
1. The MacBook Pro computers that have dark grey covers are new.
2. Leap years, which have 366 days, contain an extra day in February.
3. The English lessons that are held in Florence start at 1pm.
4. We love spending our holidays at my aunt’s house, which is in the middle of nowhere.
5. My bike that has a broken seat is in the garage. The new bike is in the basement.
6. One of the weakest teams beat the defending champions last week, which was a surprise to everyone.
7. You can give me the book that my sister lent you last week. I will see her tomorrow for lunch and I can give it back to her for you.
8. Animal Farm, which was written by George Orwell, is a great book.
9. The girl that you saw at the marathon with a huge sign is Paolo’s wife.
10. The gas tanks, which are on the first floor of the hotel, have to be replaced by December 31st.