Both «shouldn’t» and «cannot» are possible translations. «Нельзя» covers a range of meanings from 'it’s forbidden' to 'it's impossible'.
This is coming from the Wiki. "Forms «неможно» or «не можно» are non-existent in standard Russian. To negate, use «нельзя» instead." On the other hand, the predicate short form of должный is used with the infinitive verb to show that one should or must do something. For example «Мы должны всё знать.» would mean "We should know everything."
It is telling me the correct translation is: "1 can't know all." That is a horrible translation and should be deleted. It should never include a number to represent the word "one" and It doesn't sound natural. An English speaker would say: "One can't know everything" as it is stated in the translation at the top of the page. I don't know why the exercise would suggest a translation that is different from what is on this page.
So can всё mean everyone in this context?
No, that would be «Нельзя всех знать» or «Нельзя знать всех».
Also, всё is the accusative neuter of весь ?
Yes, something like that. 'Everything' is somehow uncountable, while 'everyone' is not.
Well... Нельзя can sometimes be used to talk about things that are impossible because they are forbidden, but «Нельзя всё знать» would be understood to mean 'you can't know everything'.
If I wanted to say that it's forbidden to know everything, I'd say «Всё знать запрещено», using the word «запрещено́» 'forbidden'.
While it’s possible to use «нельзя» to mean something is not allowed, I can hardly imagine this meaning used in this sentence. There’s little point in forbidding a thing that is obviously impossible anyway.
If I wanted to convey the meaning of ‘is not allowed’ here, I’d probably re-phrase it with запреще́но ‘forbidden’ or something.
"You cannot" is the closest working translation, fairly ambiguous about whether it is impossible, immoral, illegal or what. But it will always be a generic "you", i.e. it refers to people in general. This fact makes this translation both good and at the same time too vague. In English only context helps you understand the subtle difference between "You cannot speak Finnish", "You cannot outrun the arrow" and "You cannot piss onto people's doors". In Russian the first will never use нельзя, unless speaking Finnish has been outlawed recently.
If you have to express that someone is unable to to something or is not permitted to do something, there are other ways to do that.
- нельзя applied to a person is always about it being wrong rather than impossible (you should not do that).
- мочь is a verb that expresses ability to do something, used like any other verb (я могу, ты можешь etc.). Often used for one-time actions.
- уметь is a verb that expresses possessing a skill to do an action. Normally used with an imperfective verb (e.g. Он не умеет писать).
- other phrases also exist (e.g. "способен" or "в состоянии" than mean "able, capable")
If something is forbidden, запрещено or запрещается specifically expresses that (as you can imagine, that rarely comes up in speech). Their even more official versions are воспрещено and воспрещается (as in "Проход без предъявления пропуска строжайше воспрещается" ~ "Entry without demonstrating an ID is most strictly prohibited").
You can also use не разрешается and, maybe, не разрешено. Wait... you probably cannot but native speakers may do that in formal lists of rules (where "it is not allowed to do so-and-so")
Oh, мочь and можно are also used to express what "may" expressed in English. If your lose your faith in humanity each time you hear "Can I go to the bathroom?", speak Russian and be at peace. We have these words happily used in both meanings, albeit the common usage of можно and может is different.
We typically ask for permission with можно:
- Можно войти? = May I come in? (Могу я войти? is more stiff, though also correct)
We express our hypotheses about what a person may do with the verb мочь:
- Она сегодня может прийти раньше. = She may come earlier today.
- Он может не согласиться. = He may disagree.
If we express a thing one might do without specifying the agent, we often use можно:
- Можно пойти завтра. = (We) can go tomorrow (i.e. going tomorrow is a possible choice given the circumstances)
Is it better in general to translate нельзя as “саnnot”?
That really depends on the context. ‘Not allowed to’ is often OK, but not in this sentence. Because, well, how can you be allowed to know? It’s not something that can be forbidden, really, at least it Russian.
Also is there any way to differentiate “I cannot...” from “you cannot...” and “one cannot....”?
Yes, you can add the person who can’t be something in the dative-case form:
- мне нельзя́ ‘I can’t, I’m not allowed to’,
- тебе́ нельзя́ ‘you can’t, you’re not allowed to’ (informal singular),
- ей нельзя́ ‘she can’t, she’s not allowed to’,
- ему́ нельзя́ ‘he can’t, he’s not allowed to’,
- нам нельзя́ ‘we can’t, we are not allowed to’,
- вам нельзя́ ‘you can’t, you’re not allowed to’ (formal or plural),
- им нельзя́ ‘they can’t, they’re not allowed to’ (formal or plural),
- сестре́ нельзя ‘my sister can’t, is not allowed’.
Literally, «мне нельзя́» is like ‘to me, it’s not possible/allowed’.
Depending on the context we could imagine someone saying "it is forbidden to know everything" if it refers to a specific set if things (unless всё represents the great big everything only?), e.g. "for mortals it is forbidden to know everything, only the Lord can know everything"? I'm not religious but that's an example that sprang to my mind...