Should I use "was" or "were" here?

I’d go with “were”.

Was.

Was. It is collective, but it is a single group.

Was is past tense, were speculates.
If I were a rich man….
I was poor, now I am rich.

The causes… were?

@Trillian Yes, that too, the past tense.

Was !!!

This is quite boggling, they both sound correct. I’d change the sentence.

You could technically also say “is”.

Were is not correct.

Was.

The subject is “cause.” Singular. It takes a singular verb. And it’s simple past tense, declarative.

All the rest is fog.

@Jeruba I was waiting for you to reply to this as I was not sure which one was correct myself (though I was leaning toward “was” because I thought subject-verb agreement would make “cause” take a singular verb.) I love grammar questions but some of them are head-scratchers.

I was going for the past tense. I think most people here say “was” should be correct.

@muppetish, you’re correct. Problems of agreement between subject and verb disappear once you’ve correctly identified first the verb and then its subject.

No one is confused about what “singular” and “plural” mean. They get confused about which things to make singular and plural. Once you’ve found the right ones, it’s a simple matter to choose the proper form.

The issue here is not tense; both “was” and “were” are simple past. One is singular and the other is plural.

To me, this sentence seems easier than this one:

“The cause of the 2008 financial crises was/were the credit default swaps”.

That always confused me.

@Blackberry and @Jeruba was right. If I was you, I were going to say, take their advice. I sure does.

@DominicX, no change. The subject is still “cause.” What’s in a prepositional phrase doesn’t matter. It can’t be the subject of a verb.

“What was the cause of the 2008 financial crises?”
“The cause was the credit default swaps.”

Find the verb or verbs first. Then find the subject of each one. Then make each verb agree in number with its subject. That’s how you do it.

I don’t want to quarrel with @Blackberry, but there’s no collective noun here. “Cause” is a plain old singular noun.

Here’s a problem: {“Credit Default Swaps” includes } This should be “include.”

@Jeruba I thought @Blackberry was referring to the noun phrase, “credit default swaps” which is collective. And @perspicacious is right that if only credit default swaps are listed, then the word in the parenthesis should be include and not includes.

@weeveeship It would be useful to see the entire sentence.

@ETpro, “credit default swaps” is not a collective noun. It’s simply a plural. A collective noun has a singular form (“club,” “flock,” “team,” “family,” etc.) but refers to an entity that consists of more than one member and is therefore plural in concept. The verb form used with a collective noun may be singular or plural in various circumstances and according to various rules. But that’s irrelevant; we don’t have one here.

The expression pointed out by @perspicacious in the question details really means “The term ‘credit default swaps’ includes . . ” Even if that term is used collectively to refer to a variety of financial instruments, it is still in the predicate and is not the subject of the verb.

If the sentence said

    Credit default swaps were the cause of the 2008 financial crisis.

we would definitely have a plural subject, and the verb would be “were.”

@Jeruba As usual, you are the editor. You’re right.

“Was” because the credit default swaps were collectively one thing.

Thanks for clearing that up, Jeruba.

Were.