Qual è la differenza tra I bought a hat , which was too big for me. e I bought a hat which was too big for me. ?Sentiti libero di rispondere con un esempio.

Commas are funny little things that can be very important to grammar. The meaning of your sentences stays the same. However, there are times when a comma will affect the meaning. For example:
1) I like cooking, Grandparents and children.
2) I like cooking, Grandparents, and children.
Sentence 1 suggest that you cook your grandparents for dinner. Sentence 2 is just listing 3 things you like. The general rules for commas are to use them after: yes or no, a person's name, around a prepositional phrase, or when listing something such as this list here or the list in sentence 2.