The horizontal speed of a projectile launched over flat ground will affect the time that the projectile will stay in the air is this true or false?

this is false. the horizontal speed of the object has absolutely nothing to do with how long it takes to fall. if you fire a bullet on level ground it will hit the ground at the same time an Apple would if you drop it from the same distance above the ground at the same time. the force of gravity is the same on everything on earth.