Traditional grammar teaches that "would V" is paired with a superficial past tense in conditional sentences of type (b). But nothing is said about sentences like (a), where modal "would" is paired with a present tense verb in the if-clause. If both sentences are okay, what does (a) mean, and how does it differ from (b)?
a. I think the balloon would burst if you continue to fill it with air.
b. I think the balloon would burst if you continued to fill it with air.