Translation:Matěj always studied less, and now he knows less.
I think that they mean... Matěj studied less than others (classmates). If you wrote Matěj never studied much... This means; Matěj nikdy moc nestudoval. And it is different than; Matěj studoval méně. Czech language is very hard. I'm Czech person. It is my nationality language. I could help you if you don't know anything. Just text me
Yes, but learn can translate to the imperfective učit se, and the English phrase "Matěj always learned less" refers to something that was happening repeatedly. Although "studied less" is definitely better and should be kept as the main translation, "learned less" is also ok and doesn't always translate to naučit se.
After a fairly long discussion with my Czech teacher I understand it now and agree with you. So I would like to summarise the definitions here, which I think will be beneficial for other students.
učit se - "sedět u knih" rather emphasises that you are spending time with textbooks, trying to again new knowledge regardless or not if you actually gain new knowledge.
naučit se - acquire knowledge
Thus - "učil jsem se celý den, ale nic jsem se nenaučil" - I studied all day but didn't learn anything
so the phrase "Matěj se vždy míň učil, a teď míň umí." means that Matej studied less (spent less time studying) and doesn't specify how much (new knowledge) he learnt
Although - in the present tense "učím se Češtinu" means "I am learning Czech"
and a quick side note the verb "studovat" should only be used for subjects / courses Studuju Češtinu - rather implies studying Czech studies at a university. Similarly "studuju medicínu, matematiku ... " " I am studying medicine, mathematics..." but never !! "studoval jsem Čestinu o víkendu" !! In English you can say I studied Czech on the weekend but in Czech it has to be "učil jsem se Čestinu o víkendu"