aqua

Aqua, aquae is a first declension feminine noun and about as regular as Latin gets: aqua, aquae, aquae, aquam, aqua, and in the plural aquae, aquarum, aquis, aquas, aquis. If you want a model noun to check the first declension against, this is a better one than the usual puella because you will actually meet it.

The plural is where the interest is. Aquae often means not "waters" in general but a spring or a spa, and it names towns: Aquae Sulis is Bath, Aquae Mattiacae is Wiesbaden. Aquae ductus, run together as aquaeductus, is the aqueduct, literally the leading of water. Aqua et igni interdicere, "to forbid someone water and fire", was the formula for banishment, since a man cut off from both could not live in the community.

It descends whole into Italian acqua, Spanish agua and Portuguese agua, and into French eau, which is the same word worn down to a single vowel sound. English took aquatic, aquarium, aqualung and, through French, ewer.