Page 169 - Basic Japanese

behave the way you would expect a vowel verb ending in -
iru to behave:

The verb ik-u ‘goes’ is irregular only in the way the stem
changes the -k to -t instead of -i before the t-endings. (We

might have expected a form like *i-ita instead of the actual

it-ta, if the verb were regular.) There is one other verb that is
irregular in the imperfect only. This is the verb i(w)-u ‘says,

tells.’ We write this form iu, but it is often pronounced yū.

Note that the perfect and gerund forms of iu ‘says,’ iku

‘goes’ and ir-u ‘is necessary’ are the same: itta, itte. You can
tell them apart only by the rest of the sentence. (But some

people pronounce yutta and yutte for ‘said’ and ‘saying.’)
There are some verbs whose masu-forms lacks r. For

example, the polite imperfect form of kudasaru is not

kudasarimasu but kudasaimasu. Similarly, the polite imperfect
form of irassharu ‘to exist’ is not irassharimasu but irasshaimasu.

4.6. Adjectives and the copula

Adjectives in Japanese end in:

-ai like akai is red

-oi like aoi is blue

-ui like warui is bad