This can give you important information about their hands & assist you in deciding which cards to keep & which to discard. You can concentrate on gathering those cards for your own melds if you observe, for examplerummg, that an opponent discards hearts a lot, which might mean that they are not attempting a heart sequence. A further successful tactic is to keep your hand flexible. Consider keeping a variety of cards that can form several possible melds rather than obsessing over a single meld or combination. You can change your plan of action in response to the cards you draw & the moves made by your opponents thanks to this flexibility.
jvbaa