what will happen to my car if i turned off my engine while my gear was still in drive? thanks.

Supposing you're talking about a car with an automatic transmission, nothing really... It just won't start again before you put it in neutral or park. Same thing for a manual by that matter. Worst thing that can happen is if you're not parked on level ground ;) the car might move

Unless you are going down the road , and "coasting". The problem being the trans is turning , and the motor is off , the converter is not turning the pump for the trans , so now you have no lube for those turning parts in the trans . Could heat up and burn "o "rings, lip seals and even clutch plate. This is why it is suggested to remove the drive shaft or tow the car with the drive wheels off the road , if you are towing for a distance.