Which word signals a nonrestrictive clause in a complex sentence? that which while yet

The word which signals a nonrestrictive clause in a complex sentence is the following one:

which.

Explanation:

Nonrestrictive clauses are the ones that merely add information about the subject of a sentence. If they are omitted, there is no change in meaning.

For example:

Cell phones, which are very popular nowadays, can cause addiction.

In the sentence above, if the underlined part is omitted, there is still a full meaning: Cell phones can cause addiction- only the additional information is not there.

On the other hand, check out the sentence below:

Cell phones which have low storage space are dodgy.

If we omit the underlined part, the sentence has its meaning changed, since  if we say "cell phones are dodgy", we mean one should avoid ALL cell phones, and not only the ones that have low storage space. It is a restrictive clause, even if "which" is being used.

So, what counts is not so much the word that is used, but how it is used. If there is a comma, what appears next- if which, for example, is used- is a nonrestricitive clause.

However, the other alternatives must be ruled out, since they are not only about extra information; in fact, they are essential to convey a certain meaning. Therefore, they are restrictive.

Examples:

The car that I bought ten years ago in in pristine condition. (emphasis on the fact I am talking about a specific car)

I did my homework while I listened to music. (emphasis on the time of the action)

I haven't finished my dissertation yet. (emphasis on the fact that an action is not completed yet)