So how would you translate "Wierzyłaś nam?" Is it better when two different sentences are translated in the same way? Or maybe it is beneficial for learners to underline some difference? The issue has already been raised. You can read this discussion where immery provides some biblical quote: https://www.duolingo.com/comment/13389795
Part 1. Hi. That's going to involve a bit of work. There seems to be a problem with that discussion in that it seems to totally ignore the fact that not all verbs in English function in the same way, and that "believe", in particular, is used in very specific ways. This is perhaps an inherent problem with the translation method of language learning.
I haven't read it all yet, but for starters, being a state verb, "believe" is not used in Continuous, so "I was not believing" is not possible. (Although it's OK in Indian English).
We don't normally use it in Present Perfect in affirmative statements either. But questions with "ever" and negatives with "never" are possible:
"Have you ever believed us?"
"You have never believed us"
And in some examples on that page where Present Perfect is being used, we'd in fact use Present Simple. In fact "believe" is overwhelmingly used in Simple aspect.
As for the biblical quote:
Then Jesus told him, “Because you have seen me, you have believed; blessed are those who have not seen and yet have believed."
Firstly it's in biblical language, which is not necessarily a good model for everyday English, and secondly "believe" is being used intransitively here (without an object), which does not happen in normal English except to say that you have religious faith, and here again it tends only to be used in Simple tenses.
So I think the answer to your question is simply "You believed us" or possibly "You used to believe us". Here's an example from that discussion:
*"Nigdy nie wierzyli córce co do jej chłopaków" *
We have, I think, three options, but we need to add a couple of words for it to make better sense in English:
"They never believed (what) their daughter (said) about her boyfriends."
In the second option we substitute "never used to believe" for "never believed", and in the third "used to say" for "said". (But we wouldn't usually use two used tos together).
I'll perhaps add a bit more when I've had a better look at that page (which needs a lot of correcting from the English point of view, I'm afraid).
But yes, some were putting forward Continuous on that page. How to tell the difference? First of all, I have to say that I'm not an expert on the Polish usage, so I'm guessing a bit, but in a word, as with almost all translation - context:
"When she told him she was leaving him he didn't believe her"
Specific event at a specific time - "uwierzyć"?
"When she was very young she always believed her sisters when they told her it was Santa Claus who brought her all those presents
Permanent state or repeated action in the past - "wierzyć"?
But most important, people should realise that the verb systems in the two languages are very different, and should try to think in the verb system of the target language, try to feel it, not translate tense for tense from their own. In fact I try to ignore the more technical explanations in the comment pages, and just note what Polish verb forms are used in what contexts - you can sometimes have too much grammar. And that's coming from somebody who used to write a grammar blog!
With my Polish students of English, Polish tenses are rarely mentioned (everything is in English), except that with lower levels I sometimes point out that with action verbs dokonany tends to equate to Past Simple, and niedokonany often equates to Past Continuous. But it's not a perfect fit, and we don't include Present Perfect (the most difficult tense for Polish learners to get their heads round) in this distinction, as the concept behind Present Perfect doesn't exist in Polish, just as the concept of perfective and imperfective doesn't really exist in English.