What do I bid when the opening bidder bids my bid?
E.g. bids 1NT and that is what I want to bid and I don't have a 5 card suit.
There are several common situations with different handlings:
Opener to your right bids your best suit and you have 12+ points:
Pass and hope partner can make a balancing double, which you can then convert to penalty by passing. Update The rationale in this case is that opponents, if partner is bust or close to it, actually have sufficient strength to bid game and perhaps make it. Let them discover for themselves that the breaks are bad and that you are end-playable, hopefully too late to salvage a plus.
Opener to your right opens a weak 1NT and you have the same hand:
Pass; unless partner can act in the balancing seat you are more likely to be plus by defending 1NT then by attempting to make a higher contract.
Opener to your right opens a strong NT and you have the same hand:
It is common to double with this hand though many experts prefer to Pass as above. It is not clear to me that either approach is better in the long term than the other. The increased points held by you and opener make it much less likely that partner can reopen, but you may be able to make 8 tricks in partner's longest suit even when he has only 5 or 6 points.
Opener to your left opens your best suit, you have 10+ points, and you are put in the balancing seat:
Partner with likely shortness in opener's suit could not act, and so is weak. Either the opponents have underbid, or they are in trouble; either way you are best to let the auction die. The hand is either a misfit for both sides or opponents have failed to find their fit.
Opener to your left opens 1NT (either weak or strong) and you have the same shape and announced range:
Pass, especially if you have methods allowing entry over a 1NT opening to show various unbalanced intermediate hands. Nobody is getting rich on this hand, except by doubling over-aggressive opponents. Take your Average Plus result.
The fastest way to improve your scores is to become a better defender - by practicing your hand counting, your leads, your defensive signalling, and finally by practice. There is often much room for error by both defenders an declarer on these balanced low part scores. Taking one trick more on defence than your peers will often get you a very good score. Enjoy the opportunity of practicing your honed skills.
If the opponents are halfways decent they will declare as often as you and your partner, so you will be defending twice as often as you declare. Repeat that thought; again. Practice your defence, and practice your defence again. Any delusion that you can win at this game through constant sacrificing against their contracts is just that - a delusion.
In most cases, the best course of action is to pass. If you pass when you have opening values, this is known as a "trap pass."
Your (right hand) opponent (RHO) bid, and stole the so-called "first mover" advantage. This is an advantage on offense. The opponents have a head start on declaring, which is not necessarily to your disadvantage, because they may get too high if you keep silent and prepare to play defense.
By passing, you try to preserve the right of "last action" (ultimately you hope that it will be your decision to pass, bid, or double). This is an important advantage on defense. Right now, you know that your opponent has an opening hand, and wants to play in no trumps, but s/he doesn't know the same about you. You also have the advantage that your RHO is "onside" finesses that may go into your hand, which may be an unpleasant surprise for this opponent. Don't give up this advantage at this point by bidding.
The reason you pass, is because you want to hear from the two partners. If RHO has 12-15 points and you have about the same, the other two players have only 10-16 points between them. There are three main possibilities:
1) Both partners are fairly weak (each has about half of the outstanding points, or 5-8 points).
2) Your partner is fairly strong (has most of the 10-16 remaining points), and your opponent's partner has "nothing."
3) Opponent's partner is "strong" and your partner has nothing.
You will be able to tell the situation by how the partners bid. Basically, with you and RHO opponent finely balanced in strength, you don't want to take action until you hear from your partner, or at least RHO's partner.
The time to take action (or not) is when the smoke has cleared.That's why I say "pass for now."