英文文法問題，urgent！！

He hypothesized that the truck was not working because of the broken filter.
將a 轉做 the

both are grammatically correct. but changing "a" to "the" is changing the audience. a broken filter is talking to somebody who doesn't know the situation. the broken filter is talking to somebody who knows about the situation or knows about trucks.

老友，你的英文不太像英文wor，請加把勁吧。
He suggested a broken filter as the possible cause of the truck's failure.