When I have to decide whether a clause is restrictive/non-restrictive I typically figure out whether the information in the restrictive clause is, in my opinion, necessary. Is this correct or is the fundamental definition that the sentence would grammatically be incomplete without the restrictive clause. I saw some examples where the sentence would be complete without the corresponding restrictive clause, but would like to have a definite answer. Example sentence is:
Would you lend me the video that you told me about last month?
In my understanding, the sentence would be grammatically correct without the restrictive clause (“would you lend me the video?”), and thus, the definition of a restrictive clause depends on the writers judgment!
A nonrestrictive/non-essential relative clause may be omitted if it doesn’t change the structure or the meaning of the sentence. Leaving out [that] you told me about last month yields a grammatical sentence but omits an essential feature:
“Would you lend me the video?”
My immediate response is going to be
The relative clause is essential to my knowing which video you mean, thus is restrictive.