When you open the bidding with 1 of a suit and both your left hand opponent and your partner PASS, the one thing you know for certain is that partner does not have 6 HCP and will most probably not have any kind of fit with your suit. Even with 4 or 5 HCP points partner might well decide to give you a weak raise with 4 card support, just to get in the opponent's way. Now when your RHO bids you should get out of the way and pass with most flattish hands like this
Now you should look to take your chances in defence of whatever contract opponents land in. They now have the advantage because they also know your partner is very weak and should play you for the outstanding high cards. On the other hand if your hand is distributional you should bid again with something like:
Fewer points but better shape.
Again, if you have a competitive auction where both sides are bidding but one opponent is particularly pushy it's probably because they are very short in your suit. The corollary therefore is that the other opponent is marked with the length in your trump suit and you should plan the play of the hand accordingly.
Similarly if an opponent is very quiet even though his partner is bidding in a competitive auction, you should deduce that they have no fit in their suit and so the side suits will probably break badly for you if you become declarer.
It's also true that if both opponents bid against you in a pre-emptive fashion and you hold 4 of their suit, partner will almost certainly be void in that suit, with a possible singleton at most. This should influence your high level decisions towards bidding on in a tight corner.
There are lots of little inferences around during the auction. Listen to them. Make use of them. Do NOT, whatever you do, decide that as you don't have much in your hand you can just go to sleep. If you do, you will lose endless match-points as a defender. Always concentrate on the auction - even with a Yarborough.