Punctuating adjective clauses can be tricky. For each sentence, you will have to decide if the adjective clause is essential or nonessential and then use commas accordingly.
Essential clauses do not require commas. An adjective clause is essential when you need the information it provides.
Look at this example:
The vegetables that people leave uneaten are often the most nutritious.
Vegetables is nonspecific. To know which ones we are talking about, we must have the information in the adjective clause. Thus, the adjective clause is essential and requires no commas.
If, however, we eliminate vegetables and choose a more specific noun instead, the adjective clause becomes nonessential and does require commas to separate it from the rest of the sentence.
Read this revision:
Broccoli, which people often leave uneaten, is very nutritious.