sorry my mistake. mothu -> motsu.

japanese verb or adjectives change themselvs by the word connecting to them like do, does, did in English. mochi masu mothu masu (incorrect) mothu toki mochi toki (incorrect) yomi masu yomu masu (incorrect) yomu hito yomi hito (not incorrect but sooooo rare i think) the difference between yoi and ii is a little different above. (if you asked yoi and yoku, then it is same reason) yoi and ii is special type of word, i think. and i cannot explain well using my english :P