Partner opens 1 hearts. You hold 4 cards in spades, 6+ cards in diamonds and 2 singletons (a total of 8 hcp). What is the correct bidding? I assume 1 spades. But what do you bid, if Partner rebids 1 no-trump or 2 hearts?
The following guidances will apply for any natural non-canapé system of bidding, including but not limited to SAYC, Goren (4-card majors), Five Card Majors, Two Over One, and many Precision and ACOL variants.
The importance of finding any existing 8 card major fit, particularly a 4-4 fit in a major, and of identifying misfits promptly, underlie the requirement that a hand such as you describe must respond 1 Spade. Due to the weakness of your hand (6-9 points) you are only entitled to make one free-bid (ie non-forced and non-invited response) during the auction. The specific reasons for making this bid as 1 Spade instead of 2 Diamonds are the following:
With a hand strength of 6-9 points (or 6-11 points if playing Two Over One) it is still incorrect to respond 2D over 1 Heart when holding 6 diamonds and only 3 spades. In such circumstance the correct response is 1 Notrump. It is absolutely imperative for bidding success and partnership harmony that proper discipline be exercised in the making of 2-over-1 responses to a suit opening. Such bids are immediately raising the level of the auction and, even more importantly, removing bidding room; they can only be engaged in successfully when the partnership is known to have adequate strength to do so safely.
Note that having made a 1 Spade response (to a 1 Heart opening) with a 6 to 9 point hand and 4-6 distribution in spades and diamonds, it is still incorrect to rebid 2D if partner rebids 2C. This bid again promises hand strength of 10+ points, by being a second free bid. In many systems this is stronger still by being an Artificial Fourth-Suit Forcing call.
You have been asked for a preference by a partner who has declined to show additional strength; do so. The only acceptable calls at this point are:
If partner rebids 2 Hearts over your 1 Spade response, this is showing a minimum unbalanced hand with 6+ hearts and denying 4 spades. With your singleton the partnership has announced being in possession of about half the high card points in the deck, and has found a seven card major suit fit. Pass is the only call to even consider.
If partner rebids 1 Notrump over your one Spade response, then you are in territory usually governed by specific partnership agreements. Either or both of 2C and 2D will often be conventional bids inquiring further about Opener's major suit distribution and point range. You do not have the mandated strength or major suit length to consider any of these. When not conventional they will typically show more strength (10+ points) than you possess, as again they are a second free-bid. As above, just Pass and be thankful of getting out of this mis-fit auction cheaply, Weak opponents might even enter the auction and give you the chance for getting a plus score from setting them.
It may depend upon which bidding system you've agreed to use... if you're bidding Standard American, you should bid 1S. Even if you're playing 2-over-1, you don't have enough to bid 2D. And you shouldn't bid the forcing 1NT (in that system) with four spades.
By bidding 1S, then 2D (if pard bids 1NT) I think you're describing this hand pretty well.
I feel that if pard re-bids 2H, you must swallow and pass.
Although in most systems the 'system' bid is one spade, in some systems, a case can also be made for rebidding 1NT in an effort to escape into two diamonds.
Eg when playing 2/1, after 1H 1NT, opener will often rebid 2C, and now 2D by you would show a weak hand with long Diamonds. If opener rebids 2D you can pass or raise depending on exactly how strong your hand is. If partner rebids 2H he is showing six cards in hearts, and you can pass.
You give up the chance of finding a 4-4 spade fit (although if partner is strong he might dredge up a 2S reverse anyway), but in exchange, you greatly increase your chances of finding a playable contract. Eg if you have a 4-1-6-2 distribution, you will almost certainly end up in a 7 or 8 card fit if you respond 1NT, but if you choose 1S and partner rebids 2C (which is quite likely given your hand), there is no bid you will be happy to make.
With 6-9 high card points, your main choices are to raise hearts (if you have three), bid one spade (if you have four), or bid 1NT if your hand is suitable (not an option with your lopsided hand).
You may not bid two diamonds or anything else at the level of two, because a "2 over 1" bid shows 10-12 points and an "invitational" hand (to game in a suit or no trump), if partner has more than a minimum.
Playing weak "jump shifts," you may jump to three diamonds on six or seven diamonds, basically as a pre-emptive move, on something like 3-6 high card points. This is a "terminal" call that discourages partner from seeking game. With eight high card points opposite an opener, your hand is too strong for such a preempt, and too weak for an invitation, so make the available one-level suit bid, which is one spade.