"그와 그녀는 압니다."
Translation:He and she know.
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! :)