what happens if i put a apple in a plastic bag?!

If you carry it in a plastic bag for a day, and then eat it, nothing will be different.

However, if you keep it wrapped in a plastic bag it will decay faster.
The ethylene gas let out of its cells as it goes through the decay process would get trapped inside the plastic (impermeable) bag and would cause an increase in the speed of decay.

You have an apple in a plastic bag and it can now be conveniently carried from one place to another. You could put other food in the plastic bag with the apple and have yourself a bagged lunch.

the apple will very gradually start turning brown as it will not have oxygen. It will no longer taste its best.

You now have an apple in a bag.
And it will stay in there until someone takes it out.

You have your self a bagged apple