can a thief backstab with a short sword or long sword?
when i click on the attack icon it sais "leaving shadows" does the backstab mod
work then?

IIRC the thief needs a piercing weapon to backstab, so a short sword could do it but not a long sword. Not sure, though.

A backstab can be made with any melee weapon from the thief list, but ONLY melee weapons from the thief list because a backstab must be a melee attack
IF memory serves this means a backstab can be made with clubs, daggers, long swords, short swords, quarterstaff and katanas.
Has anyone ever tried to backstab with a quarterstaff?

If it's melee and a theif can equip it, then he/she can backstab with it.

Yes, and it will work, although Memnoch has disagreed with me on this in the past. The only thing is that the staff must be usable by a single class thief - MU/thieves can't backstab with the Staff of the Magi, for example.

"stabbing" is a word that has something to do with sharp pointed objects. In IWD, I usually give my thief a dagger to use in stabbing. most daggers have the ability to multiple the damage of stabbing, which means that they are for that purpose. definitely it's so ridiculous to backstap using a quarterstaff... I prefer knocking it on a stupid skull.

Whar are the pre-requisites to a successful backstab?

1. Thief must be stealthed or invisible.
2. Thief must be behind the victim.
3. Thief must use a melee weapon that a single-class thief would be able to use.
4. Have I left out anything?

Is it possible for a thief who fulfils the conditions above, miss the backstab but stillhit the target (ie does onyl normal damage)? Or would a successful hit be an automatic backstab?

What about backstab with Carsomyr +6??

Carsomyr +6 is not a weapon that a thief can use unless he has use any item , and that doesn't count for backstabbing.

Actually, the Staff of the Magi is a valid Backstab weapon--all Staves are. It's the type of weapon, not the weapon itself, that must be usable by a true-class Thief.

Weapon types not usable by Thieves (Axe, Mace, Halberd) cannot be used to Backstab. If you're a Fighter/Thief and you try to Backstab with a Bastard Sword, you'll get the message "Your weapon is not suitable for Backstab." Even if you use the Assassination ability, you get the same message.

I've heard that in BG2 without ToB installed, Stalkers were able to Backstab with any weapon, even 2-Handed Swords. This was corrected in ToB, however.