To what extent should the translation respect the grammar?

[Ruby Novacna]

...you had omitted essential parts...

Koen, this sentence had its essential parts; its verb and its subject. It is true that I would have done better to replace Athens with Paris or London instead of the ellipsis.
The question that concerns me is precisely how to define the essential parts in the paragraphs of our manuscript?

there is a much bigger problem: what is grammatical in Greek, cannot be turned into English word by word

Of course. I think it was Marco who once (or several times) suggested that any attempt at translation should be presented in both word-for-word and "smoothed" translations.

if you want to be a translator, you first need to learn the language you are translating from (or be a native speaker).

I have some doubts that we have any native speakers of Latin, medieval Greek or other Sanskrit left alive.
But I am not discouraged, I continue to learn, as an amateur, of course, which is why I participate in this forum.