When Playing Doubles: Try to think of hitting your shots "under" your opponents (at their feet) or "over" your opponents (lobbing over their heads to get them off the net).
By keeping a high percentage of your shots at the feet of your opponents you will give your team more opportunities to hit down. Usually, the team which receives more high balls in doubles wins the match. Also, by keeping the ball at your opponent's feet you minimize errors left, right and long.
When you are playing a team in which one or both partners are crowding the net and you are not able at this point to get the ball down, that is when a lob will help.
When Playing Singles: Think of moving your oppnent laterally (left to right, right to left, etc.). The concept of hitting the ball behind your opponent (hitting where they are coming from) if very valuable.
Some examples of moving your opponent:
move them left, then right, then right again
right, then left, then left again
More points in singes are won by making your opponent change direction than any other way. Once you make your opponent change directions to hit, you will usually receive a short ball which you can attack.
Think of doubles as a "under or over" game.
Think of singles as a "left to right" game.