Dealing with slow balls

Everytime I try to play a forehand topspin, it's either the ball goes over the table (but still curves down) if I try to hit it fast and with topspin or hit it with topspin but goes to the net. 

All of these errors came from slow and long balls. I kept experimenting but keep getting errors. 

Thank you, PingSkills.

Hi Francis,

Focus on the stroke itself to start with.  Then you can think about changing the angle of your racket subtly to allow for the ball going too high or too low.  The important thing is to get the path of the stroke first and not worry too much if you are making slight errors.