[Coda]

Okay.

Potential Form

To make the potential form, start with the plain form of the verb.

For -u verbs, change the -u to -eru. For example, 飲む (nomu, to drink) becomes 飲める (nomeru, to be able to drink).
For -ru verbs, change the -ru to -rareru. For example, 食べる (taberu, to eat) becomes 食べられる (taberareru, to be able to eat).

The new verb is always a -ru verb, so the negative potential forms are 飲めない (nomenai, to be unable to drink) and 食べられない (taberarenai, to be unable to eat). The normal polite formations for -ru verbs apply too, so you might have 飲めます or 食べられません.

There are two exceptions to the rule:

The potential form of する (suru, to do) is 出来る (dekiru, to be able to do).
The potential form of 来る (kuru, to come) is 来られる (korareru, to be able to come).

[Aside: While 行く (iku, to go) is considered an irregular verb like suru and kuru, it is regular here -- its potential form is 行ける (ikeru, to be able to go.)]

Edit: So when you hear someone say "yamerarenai!", you can see that they mean "yameru + -rareru", meaning "I can't quit!" or "I can't give up!"