The imperative form of Japanese of verbs is constructed differently depending on whether the verb is a -ru, -u or irregular verb.
For -ru verbs, the stem remains unchanged. For example, to make the imperative form of 食べる for example, simply use the same stem as in the other forms, followed by the verb ending ろ.
食べ・る → 食べ・ろ
With -u verbs, a new stem is made by changing the vowel sound of the last hiragana of the plain form from an う character to the corresponding え character.
書く → 書け
As we saw with the -nai ending, verbs ending in う itself can behave differently. e.g:
買う → 買わ・ない
However, with the imperative, things are simpler, as verbs ending in う behave just like the others.
言う → 言え
The imperative form of する is しろ.
The imperative form of 来る is 来い（こい）.
行くfollows the same rule as for -u verbs, with an imperative form 行け.