Subject or Object of the adjective clause?

●If there is a subject after a relative pronoun, then it is the object.

e.g. The high school which she attended is in NYC.

●If there is NO subject after a relative pronoun, then it is the subject.

e.g. Women who get married sometimes change their last names.

●If the relative pronoun is the object of the clause, it can be removed. (which, who, whom, or that)

e.g. The school which she attended is in NYC.
The high school she attended is in NYC.  (✔)

●However, if the relative pronoun is the subject of the clause, then it cannot be removed. (who, which, or that)

e.g. I found a website that lists my classmates.
I found a website lists my classmates. (x)