"She was doing experiments but she did not understand the results!"

Translation: Ea făcea experimente, dar nu înțelegea rezultatele!

[Mondori]

Mine was marked wrong only because I included "ea" in the second clause. Ridiculous!

[Pat733327]

I agree with Mondori. I put "ea" in the second clause and it was marked wrong. I think it ridiculous, too.

[JuliaBarron625]

Me too. Boring!

[cheerfulcharlie]

Why can I not say: Făcea experimente dar nu a înțeles rezultatele?

This seems to correspond to the English translation that Duo provides: "She was doing experiments but she did not understand the results!"

Is it because some verbs like "to understand / a înțelege" are only used in the imperfect tense and never in the "Perfect Compus" (i.e. passé composé)? (maybe this is the case with the verbs: to know, to see, to understand)

[coto.i]

"Făcea experimente dar nu a înțeles rezultatele." should be accepted. Please report it.

In the Romanian translation, we can use both the perfect compus and the imperfect.

It is English where a number of verbs are not used in the continuous form ("-ing" ending), among them those mentioned by you. They are called Stative / Non-Continuous / Non-Progressive Verbs. More details on this:

https://learnenglish.britishcouncil.org/en/quick-grammar/stative-verbs

https://www.thoughtco.com/non-continuous-verbs-1210762