<you> and <one>
As subjects, either can be used:
Can you/one camp in the forest?
As object, <you> is the normal pronoun:
They fine you for parking offences.

<you> is more common in ordinary conversation. It is a more "friendly" pronoun and implies that the speaker can imagine himself in such a position.

<one> is more impersonal and less often used, though the possessive <one's> is quite common:
It's easy to lose one's/your way in Venice.

The correct possessive form must be used:
One has to show one's pass at the door.
You have to show your pass at the door.

If instead of <one> or <you> we use a singular noun, the possessive adjective will obviously be <his> or <her>:
One must do one's best.
A traveller has to guard his possessions.

<they>
<they> is used as subject only. <they> can mean "people":
they say = people say, it is said
They say it is going to be a cold winter.

<they> can also mean "the authority concerned", i.e. the governmnent / the local council / one's employers / the police etc.
They want to make this a one-way street.

Source: A Practical English Grammar, A.J. Thomson, A.V. Martinet