A restrictive element is not set off with commas. This tells the reader that the information is important to understanding the sentence.
For example:
For class a student needs a notebook that is easy to write what teacher teaches.
A nonrestrictive element is set off with commas. The comma tells the reader that the information is not essential to the meaning of the sentence.
For example:
For class the student brings a notebook, which is easy to write what teacher teaches.
There are also word groups that occur in the middle of the sentence that can be restrictive or nonrestrictive, depending on whether or not they restrict the meaning of the sentence.
For example: (restrictive)
My office which is full of books is located on the fourth floor.
He is a good boy who studies hard.
For example: (nonrestrictive)
My office, which is full of books, is located on the fourth floor.
He is a good boy, who studies hard.
In the restrictive sentence, if the relative pronoun is the object of the transitive verb, then the relative pronouns can be omitted. If the relative pronoun is the object of the preposition, when the preposition’s position is the end of the clause, then the relative pronouns can be omitted.
This is the boy whom I saw yesterday.
o. Vt.
=This is the boy I saw yesterday.
For example:
This is a question that you should pay attention to.
o. Prep.
=This is a question you should pay attention to.