S.T.A.L.K.E.R.: Shadow of Chernobyl Q&A

in the sewers how do u get past the invisible monster and the military soldiers its so hard!!!

I've found the documents in the Lab for the barman, but now I can't get out because something is holding the door, how do I get out?

I'm not getting the option to press f to open door at the traders anymore. Therefore i have failed giving him the psuedo dog tail and some other one that i cant remember right now, anyone know whats goin on?

How do you fire a vog-25 grenade frm a grenade launcher?

Half way throu the game
it crashes and it sez it has
a bug and it wont work any more

how d'you fire a gtrenade from the TUNDER S14?

I seemingly accidently killed a neutral NPL, now the whole lot at 'Bar' are trying to kill me - I can not load from before I did this except if I load from many hours before...I can not be bothered to do all what I've done up till my mistake.
Is there are way ...