Here's how to think through your responses.
With Less Than 6 Points
If you have less than 6 points, you almost certainly don't have game, so just pass so the bidding doesn't get too high.
With Support
If you have at least 3 cards in your partner's major, you know you have an 8 card fit, so you have found a fit. Now your only question is how high to bid.
With 6-10 Points
You want to let partner you have some points, but not too many. Just bid 2 of your partner's suit.
With 10-12 Points
This is considered "invitational strength", since all it takes is for partner to have a little bit beyond a minimum opener for your side to have the 26 points required for game. Bid 3 of your partner's suit to invite to game.
With 13+ Points
You know that 4 of your partner's suit is almost certainly the best place to play. However, you don't have to go right there. Bid something else (2NT with a balanced hand, or a new suit, otherwise), and let your partner redescribe their hand with another bid. This may give you enough information to determine if you should bid slam.
Remember, in this situation, your partner doesn't know that you have the major fit yet. All of the possible bids you could make is beyond the scope of this post. If your next bid is 4 of the major, your partner will now know what you were doing.
Without Support
If you don't have support for your partner's major then you still have to find what suit is best for your partnership. In general, your best bid here is your longest suit, at the cheapest level that you can.
Suits of the same length
If you are have two suits that are the same length that you are choosing between, the rule is:
- If the suits are 4-4, bid the cheapest suit. (i.e. whichever one comes soonest in the bidding sequence)
- If the suits are 5-5 or 6-6 bid the highest suit. (i.e. prefer spades to hearts to diamonds to clubs)
The caveat to the above rules is:
Do not bid a new suit at the 2 level without at least 10 points!So, if you do not have a new 4 card or longer suit that you can bid at the 1 level, and you have less than 10 points, then you should bid 1NT.
Examples
For all of the following examples, assume that your partner opened 1H.
K8532
43
987
642
With less than 6 points, just pass.
AK532
43
987
642
You don't have enough hearts to support, so bid your longest suit: 1S.
AK532
643
98
642
You have support for your partner's hearts. Show that by bidding 2H.
AK53
43
A987
642
You have two 4-card suits. Since 1S is a cheaper bid than 2D, bid 1S.
AK53
T432
A98
42
You have support for hearts, and enough points to invite, bid 3H.
AJ987
5
43
AKQ65You have two 5-card suits, bid the higher one: 1S.
7543
43
AK652
64
Your longest suit is diamonds, but since you have less than 10 points you can't bid them at the 2 level. Bid 1S since you have 4 of them.
A5432
KQ6
AJ76
4
You have support for hearts, and enough points to go to game. However, with this many points you don't have to go straight there. Just bid 1S for now. You can always bid 4H on your next bid.
754
43
AQ6532
64
Your longest suit is diamonds, but since you have less than 10 points you can't bid them at the 2 level. With no suit to bid at the one level, bid 1NT.