I have two favorites. First is stay in a closed stance (if he is left forward then you are the same and vice versa) and slide step either direction slightly off his center line. As soon as the side step is initiated, launch a spin heel kick to the chest or head. If he is on his back leg, he will have a very hard time avoiding you. He will also have to deal with having his lead leg/favorite weapon "slipped" like a boxer's jab with superior reach experiences against a shorter slugger who moves inside at a slight angle. If you hit his guard, he now has to fight for balance as you follow up with a roundhouse, punch or any other follow up that makes good common sense. It is a good gap closer for the taller or shorter man in TKD.
Second one I like comes from an "open" stance. When the front leg fighter balances on his back leg, slide step into his blind side at a 45 degree angle and unleash a rear leg roundhouse either to the body or head followed by a spin side kick to his side/ribs. If you are shorter, this is an excellent way of closing the gap, sidestepping his lead leg and hand, stepping away from his rear/power side and he now must scramble to regain his proper alignment and/or distance. His stance is crossed up. You of course stay glued to him until you deliver your flurry. I call this slipping in the backdoor. A fast, tall, front leg kicker is best approached at a slight angle around his front leg.
If he comes to you, if you are able, stay just outside his reach and see if he puts his weight on his front leg appreciably, when moving forward. If so, time a straight entry toward his front leg with a front leg attack of your own when he puts weight on his front leg. He can't kick with the front leg if he has his weight shifted upon it. This gives the opponent an illusion of superior speed, on your part, if he does not understand what you are doing to him. He just knows you are getting off the blocks first and he can't draw his favorite weapon quick enough. I call this "reeling in" as in fishing. Frustration on his part causes him to change his tactics sometimes and he now must fight your game.
Go get 'em,
white belt