Restrictive Relative Clause vs. Non-restrictive Relative Clause

These days, when going over both a student's research and teaching statements and my nephew's SOP, I've found a common grammatical problem: When a non-restrictive relative clause should have been used, they used a restrictive one.

For example,

* "...it is about the electronic third party payment system which brought shock to the existing financial institutions..."

This sentence is not grammatically correct, because there should be a comma before the relative clause.

"...it is about the electronic third party payment system, which brought shock to the existing financial institutions..."

I used to tell my students that a non-restrictive relatvie clause is separated from the main clause by a comma or commas because it is only there to give further information. Its separation by a comma or commas from the main clause indicates that it can be taken away without any misunderstanding, while a restrictive relative clause, without any comma before or after it, is adhering to the main clause because it needs to be there to specify which is being talked about.

Compare the following sentences:

The girl who runs the fastest in my class has won more than ten medals so far.

Maria, who runs the fastest in my class, has won more than ten medals so far.