Learn Hardcore Latin: Aqua e foramine stillat. - Water drips from the opening.

In this sentence, aqua is the subject — the thing doing the action of stillat. Since Latin usually puts the subject in the nominative case, we get aqua.

Why is it foramine instead of foramen?

Because e takes the ablative case.

The dictionary form is foramen (a hole, opening), but after the preposition e/ex (out of, from), the noun must be ablative. The ablative singular of foramen is foramine.

So:

foramen = nominative singular
foramine = ablative singular

Why is the preposition e used here?

E means out of or from. It is used with the ablative to show movement or source from inside something.

So e foramine means out of the hole/opening or from the opening.

That fits naturally with stillat, since dripping happens from some source.