If a DC motor is rated 12V,300rpm. What will happen if i supply it with 9V

probably spin slow

maybe overheat because of higher current.
STRANGE, the spin keeps the current LOW

it will run slower for sure maybe not at all.. but if its cooled by a fan then it may overheat too..

we tried that with a motor to use a speed control on it and it worked but we forgot that there is a fan used to cool the motors somtimes and when we slowed the motor we slowed the fan that meant less air cooling it and it overheated..

It may not get started if the starting current is too low. If it does get started, it will run at below 300 rpm.

nothing, it wont start, voltage is to low, an the motor has diode in it to prevent this

It will rotate slower, though I'm not sure about the torque.