i have been playing Planttycoon for a week and ivé been wondering if the plant act like real plants/

dose giving a plant to much water harm the plant?

Yes. For example, overwatering spiderplants makes some of the leaves turn brown and fall off...

They behave "real" in some aspects but not in others. But yes, giving a real plant too much water can be one of the fastest ways to kill it. It causes the roots to rot and without roots the plant dies.