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.

That would be kinetic energy