"one of the things which is ..." or "one of the things which are ..."?

Question

The question is simple: Which sentence is correct?

This is one of the things which is really important to her.

or

This is one of the things which are really important to her.

When I think about it, the second sentence seems correct because 'things' and 'are' are both plural and so in agreement. But, I may unconsciously use the first one when I'm writing, say, a letter to a friend. Is my unconscious mind really wrong here?!

Answer 1

Both sentences can be correct:

This is one of the things which is really important to her.

This is one thing. It is really important to her.

This is one of the things which are really important to her.

Out of all of the things that are really important to her, this is one.

So, either one thing is important, or the things are important. In the singular meaning, the other things might or might not be really important to her.

Another difference is that

This is one of the things which are really important to her.

needs no previous referent because "things" is general enough to be understood generically, whereas

This is one of the things which is really important to her.

needs a referent for "things" because "one" makes it specific:

There are many interesting things in her life, some important, some not. This is one of the things which is really important to her.

Adding a comma makes the meaning clearer:

This is one of the things, which is really important to her.