If you want to say "he plans to teach", would it be навчити, or could you still say вчити?
Вчити or навчати української
When is "української" used?
Української is just genitive, so anytime you have to use genitive :)
Він не знає української (мови) - He doesn't know Ukrainian
Does it mean that any negative sentence should be in genitive case?
Can it be also учити? I think I read here something that в- and у- can be changed one to another in Ukrainian...
Generally one should avoid multiple vowels or consonants in a row "планує учити." That is why the two prefixes у-/в- exist. We accept "учити" in this case, but, strictly speaking, it is wrong.
Вчитися is correct if you want to say 'someone is learning'
So does вчити also mean to teach?