The irregular verbs: する and くる

English has hundreds of irregular verbs. Japanese has two: する, “do”, and くる, “come”. Every other verb follows either the RU pattern or the U pattern, without exception. That is why it pays to learn these two by heart — after that, every verb is regular.

する: します (polite), して (て-form), しない (negative, plain form). くる: きます, きて, こない. Notice that くる changes its vowel — く, き, こ — depending on the form. It is the only verb that does.

Two common verbs each have one quirk without being irregular. いく, “go”, has the て-form いって, not いいて as the rule would give. ある, “exist”, has the negative ない — not あらない. Everything else about them follows the pattern.