Learn Farsi: آب سرد است.

In Farsi, there is no word for the definite article "the". A noun on its own like آب (water) is typically understood as definite ("the water") or referring to the general concept. To make it indefinite ("a water" or "some water"), you use یک (yek), which is what you are focusing on in this lesson!

Why does the sentence end with است?

است (ast) means "is". Farsi usually places the verb at the very end of the sentence. The literal word order here is "Water cold is".

Wait, shouldn't there be an "e" sound connecting آب and سرد?

You only use that connecting sound (called the ezafe) when an adjective directly attaches to a noun, like in the phrase "the cold water" (آبِ سرد). In this sentence, the words are separated by the verb "is" (The water is cold), so they don't link together with an ezafe.

Will Farsi speakers actually say است (ast) like this?

In writing and formal speech, yes. But in casual, everyday conversation, است is usually shortened to just an "e" sound attached to the end of the previous word. So a native speaker would typically say آب سرده (âb sard-e).