im confused with factions please read below to understand me

[c0ld]

i joined the brotherhood of steel (BOS) and someone told me that i cannot go and do pvp because i joined a npc faction is itt true? also my second question is that if i quit the brotherhood of steel (BOS) will i be villified from them or can i join back later on?

[The Trooper]

You can go PvP whenever you can, but, you cannot join a Player based faction withou leaveing the BoS faction.
In case you lose karma (and you will if you leave BoS just depents to which other factions) you can easely reset karma for 5k caps in cathedral, be sure to bring 5 caps for the door keeper.

[Awgy]

I am sure gate keeper charges 10 caps but yeah What trooper said

[S.T.A.L.K.E.R]

You can still participate in town control, except you won't count as a soldier taking the town, you can stay with others defending, or attacking.

Don't know who told you that. As for 2nd question, yes you can leave and join back, just do what dende said about resetting you're reputation.

[swinglinered]

i joined the brotherhood of steel (BOS) and someone told me that i cannot go and do pvp because i joined a npc faction is itt true? also my second question is that if i quit the brotherhood of steel (BOS) will i be villified from them or can i join back later on?

It seems the BoS is bugged anyway, so just quit and reset karma.

[S.T.A.L.K.E.R]

BoS isn't bugged, it's quest just aren't finished, same for Enclave.

[Gargantua]

Bugged/Unfinished is basically the same difference for the purposes he is concerned with.