The three verb groups
Group I (godan) / Group II (ichidan) / Group III (irregular)
Japanese verbs fall into three groups according to how they conjugate. Group I: the syllable before ます ends in -i (書きます, 飲みます). Group II: it ends in -e, or in -i for a few like 見ます・います (食べます, 起きます). Group III: します and 来ます. Knowing the group is essential for building the て form.
Examples
書きます(I)・ 食べます(II)・します(III)
To write (I), to eat (II), to do (III).