Q:

“cause” or “causes”?

Here’s a sentence that I translated from Chinese.
My American friend just proofread it for me, but we had an disagreement over the word “cause”. He said that I should’ve use “cause” instead of “causes”, but he didn’t know why.
He said that it felt odd to use “causes”.
Could anyone explain it to me regarding why I should use “cause”?
Here’s the sentence:

It’s not inappropriate to focus on eliminating highly susceptible places or circumstances for fire, but the problem is: Are illegal rooftop subletted rooms the only factor that causes such tragedies?

Please let me know if there’s anything else in my sentence that’s weirdly constructed, thank you!

A:

The question at the end of that sentence sounds odd because you are mixing a plural noun (i.e., rooms) with a singular noun (factor) before the verb. 
Consider: 

Do illegal rooftop subletted rooms cause such tragedies?

In that wording, the plural rooms matches with the verb cause. 
Or: 

Is this the only factor that causes such tragedies?

In that form, the singular factor matches with the verb causes. 
Your sentence mixes the plural rooms with the singular factor, making it hard for you to figure out which form the verb cause(s) should take. (This isn’t necessarily ungrammatical, but sometimes this can make a sentence sound odd.) 
I think one easy way to remedy this is to use the noun form of cause: 

Are illegal rooftop subletted rooms the only cause of such tragedies?

but you would have to decide if that shifts the meaning of the sentence too far away from your original intent. If so, you could try removing the that, and  using an -ing form: 

Are illegal rooftop subletted rooms the only factor causing such tragedies?

