You are looking at some new FBD. You determine from it that there IS a (non-zero) net force acting on the object. (a) What MUST be true, for this and EVERY FBD, that tells you that there is a net force acting on it? (In other word, no “special cases” where say there is only one force; your answer must apply to each and every FBD with a net force)”

(b) Let’s say there is a net force to the right. What, if anything, can you conclude about the MOTION of the object, in the VERTICAL direction?
FBD means free body diagram.