Why is the verb 教える in this form?


In the sentence "スーさんは高校に英語を **教え** に行きます" [listen
here](https://clyp.it/yjbfdaaa?token=bbc2a2945f61652481eca66639492f4d)

Why is the verb to teach like this: "教え". Instead of "教えり"? I thought that
because it is a Godan verb ending in る it gets changed to り. I know it does
that when ending in ます, but why not when it is in the middle of a sentence?

I hope I haven't misheard what what said and made a fool of myself.

_edit: I listened to the next sentence in the exercise and this one doesn't do
the same as the previous sentence: スーさんは喫茶店にコーヒーを **飲み** に行きます, this is really
confusing me!_




〜に行きます must be preceded by a **verb stem** : in other words, drop 〜ます from the
polite form.

Examples:

> 教える → **教え** ~~ます~~ → **教え** → **教え** に行きます
>
> 飲む → **飲み** ~~ます~~ → **飲み** → **飲み** に行きます

