How is momentum similar to inertia? explain.

Inertia is the property of opposing an external influence or force that changes its state of inertia.  When a body is at rest it continues to so.

Momentum is mass * velocity.  As long as the object with a uniform velocity continues do move in the same direction (and with same speed), the momentum remains constant.  Momentum does not change unless there is an external force on the body forces the body to change its velocity.