This question already has an answer here:
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.