If you increase the mass of an object, but keep the force on it constant, its acceleration decreases? true or false

That statement is true.

Prof Newton said: Force = (mass) x (acceleration)

- - If the force is constant, then the right side

of the equation must be constant too.

- - The right side of the equation is the product

(mass) x (acceleration).

- - If either of those changes, then in order to keep the product

constant, the other one has to change in the opposite direction.

- - So if the mass increases, then the acceleration decreases.

It really depends. On a hill, the object would increase speeds. Flat terrain, it would slow.