From what i have learned in Basic Korean: 1. if the verb stem ends in a vowel, add N on the patchim 2. if the verb stem ends in a consonant, add "eun" 3. if the verb stem has "hada", delete "da" then add "N" or "neun" on the patchim 4. if the verb stem ends in L, delete L and put N 5. If the verb stem ends in P, delete the P then put "un" 6. If the verb stem ends in "Ss", put "Neun".

i don't wanna say this, but your question is too blur to answer. If you make a question with more specific examples, more people can answer to you.