Translation:His fifth book disappointed me very much.
I'm noticing that "really" is rarely an accepted translation of "velmi," but it seems like a more natural translation in many cases. I wouldn't typically say "it disappointed me a lot," I'd say "really disappointed me." "Really" is a pretty common generic intensifier.
So is "opravdu, skutečně" which is the true equivalent of "really". Generally, translations should be as close as possible. I am not sure what the consensus is in this specific case.
I don't think it's actually true that those are the only "true" equivalents of "really", since "really" gets used in a much broader rage of contexts. "Really" is the default intensifier in American English. It's not just for situations where you want to say "for real."
"Very" is not as frequently used and sounds more formal. And I can't remember the last time I said "this disappointed me a lot," I don't even think I'd ever say that. I'd only say "this really disappointed me."
But that is also true of the Czech equivalents! Opravdu, skutečně, colloquially fakt, fakticky, vopravdu.
"This really disappointed me." -> Tohle mě opravdu zklamalo.
"Generally, translations should be as close as possible." To a literal rendering? Word for word? That's one theory, but of limited practicality in real life situations. In the classroom, sure.
"Really" should be given as a correct translation. As a native speaker of American English I would say "really" here.
I would say as a uk english speaker 'i was very disappointed in his fifth book'. Is that wrong