If you jailbreak your iPhone 3GS can you still pay for apps?
No the data won't delete from the iPhone unless you reset everything
I have an unlocked and jailbreak iPhone 3g 16gb and i want to know how to add music to it with-out loosing your unlock and jailbreak Can you use itunes and still know that everything will be ok?
I think you could but it may be better if you get dtunes from cydia or installous mainly because it free but a bit slow.
If you jailbreak an iphone do you still have to pay the data plan?
It depends. If you UNLOCK it and put a new SIM in: No. If you just JAILBREAK it: Yes, yes you do.
You reset your iPhone 3G without copying the apps will you still be able to get them back or download them for free?
A iPhone 3g will not survive a fall out of a moving car.
Can you buy apps from apps store on your iPhone 3g when unlocked and on tmobile?
I would believe so, it can be difficult to unlock your iphone though but the iphone is exclusively at tmobile in Europe.