Generalizing all lefties into one category is difficult, but I’ll do my best to explain some general strategies that would apply to most lefties.  I’m assuming that you (the reader) are right-handed.
 
Serve Return
When the lefty is serving a forehand serve from the backhand side, adjust your position slightly more to the right.  The lefty will often use a sidespin serve to curve the ball away from your wide forehand.  By standing more to the right, you will be able to better cover the wide forehand.
 
Down-the-Line Block
When looping to the lefty’s backhand, be ready for him to block anywhere.  If you have just played a wide forehand, he will probably try to block down-the-line quick to your backhand.  Try to stay close to the table, shorten your stroke, and backhand loop to a good location.
 
Expose the Forehand
By attacking to the wide forehand with your backhand, you will usually force the lefty to move slightly back from the table.  This is exactly what you want!  By moving him back, you will weaken his backhand and force him to play with a bigger court.  Try to stay close and control the table after he backs up.
 
Higher Blocking
When playing a backhand loop against a righty’s backhand block, the ball typically stays fairly low.  When looping with your backhand against a lefty’s forehand block, the ball often comes slightly higher.  To adjust for his difference, start your backswing high and loop through the ball.  If the block comes too high, you might consider stepping around to use a forehand on that ball.
 
Spin
When a lefty is serving sidespin, it is easiest to stop his forehand serve with your backhand and easiest to stop his backhand serve with your forehand.  If you can’t use the preferred side to receive, you might need to bend your wrist back slightly to deal with the heavy sidespin.
 
Soft and Hard
This lefty probably has a better backhand block and a better forehand counterloop.  If you are stuck and forced to give a weak loop, then play to his middle or backhand.  If you are in position and can play a powerful loop, I would recommend playing 60% of the time to his wide forehand, 30% of the time to his middle, and 10% of the time to his wide backhand.
 
Other articles coming soon will talk about specific strategies against loopers, blockers, penholders, choppers, pips, and many other styles.  Apply these other strategies against the lefty for a complete game-plan.