when we sit on a still bicycle,we can't balance it but when we sit on a moving bicycle ,we can balance.why so?

Setting aside for the moment the possibility that you can balance on a still bicycle, here's why you can balance it while moving:

A bike has a steering mechanism that is built to correct for lean - That is to say, when you are moving forward, and start to fall over to the right, your natural tendency is to angle the _handlebars_ over towards the right as you do, which in turn corrects the lean by pushing you over to the left. Why? For the same reason that NASCAR tracks are all bevelled up along the curves, because of centrifugal force. (I know, there is no such thing as centrifugal force, but for the purposes of this example, it can be assumed to exist, even if it is an illusion.)

Incidentally, "learning to ride a bike" is pretty much just learning to turn INTO a fall in order to correct it. For most of us it's counterintuitive at first.

The wheels are spinning on a moving bicycle. The wheels act as a gyroscope and tend to keep the bicycle upright.It's the same action as a spinning top that is kept upright.

You can balance a stopped bicycle. Practice and you can do it. I can balance mine, and I am 50.

inertia