The verb 걷다 is used in two types either a regular or an irregular. When it refers to 'walk', it is used as an irregular verb, but when it refers to 'pull/roll up', it is used as a regular verb. So, the conjugations of each verb are rather different from each other. 1) to roll/put up (regular) 걷다. 걷-고, 걷-으니, 걷-어서, 걷-으면, 걷-어라 2) to walk (ㄷ-irregular) 걷다. 걷-고, 걸-으니, 걸-어서, 걸-으면, 걸-어라 In addition, I think you should consider the conjugation of verb 걸다(to hang, hook, put up), comparing with the aboves. 걸다. to hang, hook (ㄹ-irregular) 걸다. 걸-고, 거-니, 걸-어서, 걸-면, 걸-어라

I think it's better to know one by one. 해 is the imperative of 하다. 해요 is the honorific of 해. 했어 is the past of 하다, and so on. Those are mostly about tense and honorific except 함. 함 is a bit like the gerund or verb nominalization of 하다. I don't know what the exact term is, though. You can just add a ㅁ to the root of the word, which is 하 in this case. e.g. 알다 > 앎, 가다 > 감 etc.

It appears to be a ㄷ irregular, just like 듣다. Which conjugations do you think are regular?