That is used when important information is added. Which is used when the information added is not important.
The house that is on fire is white.
That is 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.
Which is used when the information being added is not important to the sentence. If you use which you also need to use commas.
• My kitten, which was very friendly, ran away yesterday.
Without . . . which was very friendly, . . the sentence still means the same.
• My kitten ran away yesterday.
Remember: That is important, which is extra.
The iPad that he is holding can give you a beer.