On ne sait jamais.

This is more like “you never know”, and “on” refers to the generalised/indefinite you, as opposed to a specific you, “tu/vous”.

Il faut oublier.

The impersonal “il faut” is used neutrally when the idea of “who does something” does not particularly matter, or when it is obvious. It can be “we/you/I need to forget”, depending on context.

If you need to specify the who, on the other hand, it can take the form of “il nous/me faut oublier” or “Il faut que je/tu oublie/oublies”.

In my experience: As overwhelming as it may seem at first, it will all start falling into place without the need to dwell on the minutiae and to translate them into English as you come across a variety of sentences with these expressions.