Why will my stihl string trimmer start but not run?

Your trimmer will start but not run. Very vague so we'll try and come at this from a couple of angles. First and foremost. Did you replace the in tank fuel filter? with 2 cycle this is a frequent culprit on starting and running issues due to the fuel/oil mix often gums the filters. Second: Have you looked at the spark plug? chances are its nasty brush clean with a wire brush, spray a bit of carb cleaner on it, dry it and check the gap. (gap info should be in the users manual) Will not run as in Idle. Could very easily be the idle control screw needs adjustment. this should be located on the carb very often in the vicinity of the mixture screws if the machine starts and you can keep it going with the trigger then insert a (usually phillips) screwdriver and gently start turning right until the screw catches the throttle then back off until it dies turn right 1/2 turn and test. If the unit will not run at all beyond the above then its very likely needing service. Most small 2 cycle tools use very fine diaphragms to actuate the carburetor and they notoriously get fatigued or get "crispy" over time leaving the carb needing a clean-out and new diaphragms and needles/seats installed. Good Luck