если не знаешь слов: I would expect слова - accusative plural?
слово - nominative singular слова - accusative singular
слов`а - nominative plural
слов - accusative plural
Мммм. слов would be genitive plural, not accusative
In Russian you very often use genitive in negative sentences, instead of accusative. The meaning tends to be close to “no… at all”
Не знаешь слова = you don’t know the words
Не знаешь слов = you don’t know any words [at all]
Oh sorry I mixed up genitive and accusative. You are correct there.
Anyhow, Не знаешь слова is grammatically incorrect, tho people say it all the time regardless. Не знаешь asks чего? and слова answers что?