"They were never doing it."
Translation:Nigdy tego nie robiły.
Polish imperfective verbs are often translated into the progressive/continuous aspect in English, nontheless, those two concepts are not the same. While 'nigdy tego nie robili' is a natural sentence to say, 'they were never doing it' sounds a bit awkward, to be honest. I would suggest 'They have never done it before', which imho would be an accurate and natural interpretation of this Polish phrase.