Take the verb stem and add either ㄹ 수 없다 or 을 수 없다. If the verb stem ends in a vowel then add ㄹ 수 있다 and if it ends in a consonant then add 을 수 있다. :) I'll give you some examples on how to say "I can't do ___". To say "I can't stop", then add 그치 in front of the sentence. :D Feel free to message me if you have any questions. :) Examples: 잘 수 없다= I can't sleep. 볼 수 없다= I can't see. 먹을 수 없다= I can't eat. Hope I helped! :)