Question

Earth is considered to be an inertial frame while it is actually pseudo frame. Why?

Answer 1

Over any short period of time, the tangential velocity of a particular location on the Earth (with respect to the Earth's centre) remains relatively constant. This is because a circle/sphere appears flat when you get very close to it - just as the ground appears flat. Also, the tangential velocity of the Earth as it orbits the Sun remains relatively constant over short periods of time, for the same reason.