Iknow that earth is spinning very fast but why don't we fell it spinning ?

We never feel motion.  What we feel is CHANGE in motion ... when either speed
or direction changes.  That's why it's so easy to fall asleep on an airliner cruising
at a steady 500 miles an hour.

On the Earth, the speed doesn't change at all.  The direction does change, because the Earth is spinning and your motion on it forms a circle every day.
But it takes 24 hours to describe a full circle of 360 degrees, so the direction
of your motion is changing only about 1/4 degree per minute, and that's not
enough to feel.