"그와 그녀는 압니다."
Translation:He and she know.
I think it's correct, even though I would rarely use it. They probably wanted to teach us the "he" and "she" pronouns in Korean (remember though that they are more often omitted than not).
This is super awkward in Korean since the subjects are usually omitted and you barely ever use pronouns. It might just be trying to teach you the vocab but it's important to note that Korean really doesn't function much like English at all
We'd more likely have occasion to say "he and she both know". Otherwise, it is indeed an awkward sentence that would rarely be used.
ㄹ is an irregular! It's kind of like how contractions work in English, where it's just harder to pronounce. So the ㄹ disappears and you just add -ㅂ니다 as if there was no 받침.
ㅂ/ㄷ/ㄹ are all common irregulars so it would be worth it to get a good understanding of them as you move on! :)
In formal speech, when a verb stem ends with ㄹ the ㄹ gets dropped and the verb is conjugated as if it ends with a vowel.