There are 3 groups of verbs. Group 1 verbs: (when in plain form) that end in “u” column such as u, ku, su, mu, nu and etc. These verbs require one extra step when changed to masu (polite) form. We cut off the last hiragana and then add a new hiragana in the “i” column of the same row. So, ‘su’ becomes ‘shi’, ‘ku’ becomes ‘ki’ and etc. Hanasu = becomes hanashimasu, we drop of the ‘su’, add ‘shi’ and then finally add masu. Group 2 verbs: verbs that end in ‘ru’ for example, taberu, miru. Group 2 is easy because we simply drop off ‘ru” and add masu. No need to add new hiragana. Group 3 verbs: irregular verbs, they dont follow the above rules. But there are only 2 verbs in Group 3. Kuru (to come) becomes kimasu and suru (to do) becomes shimasu. For Group 3, no magic rule, just memory.