1. Я часто слышала от Вани, что он не понимает английские слова.Originally Posted by JackBoni
That means that I often talked to Vanya, and every time he told me that he doesn't understand some English word. I have at least several conversations with Vanya. Probably (but not necessary), every time he told me about only one word.
2. Я слышала от Вани, что он часто не понимает английские слова.
That means that I talked to Vanya (maybe only once) and he told me "I often don't understand English words" (he meant many different words).