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