Singular noun + linking verb + plural predicate nominative [duplicate]

Question

Essentially, we have "[singular subject] was/were [plural object]."

Which is correct?

"What mattered was my determination and effort."

"What mattered were my determination and effort."

I understand that "my determination and efforts" can be replaced with a singular adjective (e.g. "What mattered was important."). This makes me think "was" is correct.

However, "determination" and "effort" are two words.

In the above sentence, "are two words" is correct instead of "is two words", which leads me to believe that "were" is correct.

Maybe the fact that "determination" and "effort" are not quantifiable is making the singular choice sound more "right" than the plural choice or vice versa.

Answer 1

"What mattered was my determination and effort" is correct.

Strangely, if you inverted the sentence, the correct way to say it would be "My determination and effort were what mattered."

EDIT: This is because singular subjects are always conjugated in the singular form, regardless of the plurality of the object. In my second example sentence, "determination and effort" form a plural subject and so need to be conjugated as plural, regardless of the singularity of the object.

Answer 2

Either is possible.

If you are referring to the idea of a combination of your determination and effort, you use a singular verb "was". On the other hand, if you want to emphasize both qualities separately, you use the plural verb "were".