Go(5)-dan verbs have plain forms ending in -う -く -ぐ -す -つ -ぬ -ぶ -む and -る
"Go-dan" means "five steps"
These verbs are called "go-dan" verbs because when the endings change, we use all the five different syllables from each row of the hiragana chart.
Which row of the hiragana chart depends on the plain form (dictionary form) ending of the verb. e.g. If the verb ends in く, like いく, we'll use か き く け こ in front of our endings, like this:
いかない (plain negative form - does not go / will not go) いきます (formal, -masu form - goes / will go) いく (plain (dictionary) form - goes / will go) いけない (potential form - can go) いこう (volitional form - let's go)
And so on and so on. This pattern works for all go-dan verbs.
To see the different forms of each verb, click on the examples below: