Guidelines on playing against one opponent when the flop contains a straight or flush draw.
Anytime a straight or flush draw is present on the flop and you have only one opponent, you must be aware that a draw is a logical possible holding to be up against. If you bet and are called or raised, you can expect your opponent to be holding a pair or better, or a draw to a straight or flush.
In general, if you are first to act, you should bet when you hold second pair on the board or better. Also, bet with any flush draw or an open-ended straight draw, as these hands have nine and eight outs, respectively, to improve, in addition to the chance that your bet might induce your opponent to fold. It is good to take an aggressive approach when heads up if you have hit the flop decently at all, as there is value to be gained from your opponent folding. You don't want to give him a free opportunity to catch up.
If your opponent bets, you should call him with any pair, since it is possibly the best hand. Also, call with a draw to a flush or an open-end straight, as you have enough outs to justify staying in the hand for the small bet on the flop. When you hold any of these hands, you have at least a decent shot of winning the pot, and it is important that you don't give your opponent too much credit by folding too often here. You are playing too weakly if you do.
There are several hands with which you should be raising here. First, if you hold a flush draw and two overcards, you should raise. An example of this is if the flop is 3-5-9 with two hearts and your hand is K-J of hearts. In this case, you have as many as 15 cards that can win you the pot, providing a king or jack is good. Possessing this number of outs makes it more than 50 percent likely that you will improve by the river, making aggressive play desirable.
You should also raise with a straight draw and two overcards. If the straight draw is open-ended, as in the case of a board of 4-9-T when you have Q-J, you have eight cards for the nuts and six more that may make you the best hand. This is only slightly worse than the flush draw example above, and is still a situation in which raising is a good play.
If the flop is 4-9-T and you hold K-J, you have an inside straight draw plus two overcards, making 10 possible outs. Raising is questionable here, but can be the right play if you think it will stop your opponent from betting into you on the turn. Then, if you miss your hand, you may decide to check and see the river for free. This is commonly referred to as "buying a free card," and you can utilize a raise for this reason with any of the examples shown above.
One final hand to raise with on the flop is when you hold top pair, excellent kicker, or better. Now, you figure to have the best hand, and should be charging your opponent to outdraw you. Playing aggressively when you have the best hand is one of the surest ways to add to your poker profits.
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