мочь is an imperfective verb, i.e. it is for a continuous or recurrent action. Imperfective verbs can be in the past, present and future tenses.
смочь is a perfective verb, it is for an action that has happened or/and is completed or/and has only one occurrence. Perfective verbs can be in the past and the future tenses, but never in the present.
If I may, I would suggest to look up the verbal aspects in Russian, perfective and imperfective verbs.