Who, which and that can be used when the information being added is important to the sentence.
- The dog that is walking down the street is very big.
(that is walking down the street is important because without it we wouldn’t know which dog it is.) - The car which I bought last year is the best I have ever had.
Which or who but not that are used when the information being added is not important to the sentence. The information that isn’t important needs to be enclosed in commas.
One of my favourite counties, that I have visited twice, is Albania.- One of my favourite counties, which I have visited twice, is Albania.
(Without which I have visited twice the sentence still means the same.)
One of my favourite counties is Albania.
