___ the plumber could repair the toilet, he still would not do it.
A. However
B. Even if Answer: B
I wonder why However can’t be put in this sentence, is it because of comma which should be put right after however(when it is a conj.).
You need a subordinating conjunction because you’re trying to make one clause dependent on the other, “he still would not do it.”
Using the adverb “however” would create two independent clauses joined merely by a comma. That can never happen. It’s the equivalent of “I went to the store, he followed me.” I would need “and,” for example, to make that work. “And” is a coordinating conjunction, which can combine two independent clauses, those that could function as sentences unto themselves if separated.
In short, you cannot use “however” because you would need a coordinating conjunction before the comma to make a complete sentence. Only “even if” makes that sentence complete with no additional work.
I understand the difference between them.
Thanks.
Instinct. 
I read the sentence and feel it should be “even if”. However, I think people better not to rely on their instinct when learning languages. 