Suppose your opponent has a weak, erratic backhand but a strong forehand. Suppose you get into a fast backhand to backhand rally. Should you keep going to his weak backhand or should you vary things by going to the forehand? The same question can be asked of any tactic – if something works, do you keep doing it or do you vary things so the opponent isn't sure what's coming?
This type of thinking is central to tactical thinking. It’s a judgement call. On the one hand, using the weak backhand example, you could just lock him up on the backhand side and win that way. And that’s often all it takes. (If so, don’t overthink it – simple, winning tactics are the best tactics.) On the other hand, since he’ll know where you are going, his backhand will likely get better, and his weakness suddenly becomes a strength, or at least not a major weakness.
On the other hand, if you go to his strong forehand, you get his strong forehand. What to do?
Again, it comes down to experience. Some may fear going to the strong forehand, but with experience you learn when to do so. For example, if the opponent is camped on his backhand side and gives you a slightly weak or shorter ball, you can attack that ball hard to the forehand, knowing that he’s both out of position and you are making a strong, angled shot. Guess what? Not only will he likely have trouble with that shot, but it 1) puts him out of position for the next shot that, yes, goes to the backhand, and 2) it means he can no longer camp out on the backhand, and so his backhand side becomes weak and erratic again.
Another common placement tactic is to play the middle, where the opponent has to move to play their stronger side. When you play the (in this example) weaker backhand, you should really be playing both the wide backhand and at their middle (the midpoint between forehand and backhand), making them move to cover both spots – but perhaps a little to the backhand side if you want to avoid the stronger forehand. Make them cover as much table as possible with their weaker side.
So, it all comes down to experience and thinking about these things, and then going to the right spot with each shot will become a subconscious reflex.
Also try to see if from the opponent’s point of view. For example, I always had a strong forehand but my backhand, while consistent, wasn’t very strong. And so I’d often camp out on the backhand side, just keeping the ball in play, but watching the opponent carefully – and the instant I saw him going to my forehand, I’d jump all over it since that’s what I was waiting for. Smarter opponents realized this, and they’d instead either wait for a ball to put away from my softer backhand, or simply play at such wide angles to my backhand that I couldn’t effectively cover the wide forehand off a strong shot – and if I did, my backhand would be open. Or they’d attack my middle where I’d cover with my backhand – but since I’d have little angle into their backhand from that spot, they’d be able to end the point with their forehand.
Note that while I’ve used strong forehand/weak backhand in this example, it could be the other way around. Or the two sides could be equal, in which case you’d want to move the ball around a LOT, so that the opponent can never be comfortably in position. Or it could be some other tactic – for example, if they are weak against backspin, do you give them a steady diet of backspin, or do you mix it up? If they have trouble with your slow, spinny loop, should you do that over and over or mix it up with other types of loops? If they have trouble with a particular serve, how often should you use that serve?
With experience, you’ll learn instinctively what to do, and then you’ll lock up opponents up with your tactics.