Learn Hardcore Turkish: Kap içinde su var. - There is water in the container.

Kap is a general word for “container,” “bowl,” “jar,” or any vessel that can hold something. In English you might say “container” or “bowl,” depending on the context.

Why is içinde used instead of just sticking a case ending on kap?

İçinde is the locative form of iç (“inside”) plus the suffix -de (locative). It literally means “inside of.” You could instead say kapta by adding -ta directly to kap, which also means “in the container.” Using içinde makes it explicit that you mean “inside.”

Why is there no word for “a” or “the” before kap or su?

Turkish has no articles like a or the. Definiteness or indefiniteness is understood from context. So both “a container” and “the container” can be expressed simply as kap, with other clues in the conversation distinguishing them if needed.

What function does var serve, and why is it at the end?

Var is an existential verb meaning “there is” or “there are.” In Turkish, existential verbs usually come at the end of the clause. So su var means “there is water,” and kap içinde su var literally “inside the container water there is.”

Could I say Kapta su var instead of Kap içinde su var?

Yes. Kapta su var and Kap içinde su var both mean “There’s water in the container.” Kapta is the shorter locative form. Kap içinde simply emphasizes “inside.”

How do I turn this into a question, like “Is there water in the container?”

Add mı (the yes-no question particle) after the verb and adjust vowels for harmony:
Kapta su var mı?
(Note: you can also say Kap içinde su var mı?)

How do I say “There isn’t any water in the container”?

Replace var with its negative form yok:
Kapta su yok.

Can I pluralize su (water)?

Generally no—su is an uncountable or mass noun in Turkish. If you need to talk about individual units (e.g. bottles of water), you add a classifier:
Bir şişe su var. (“There is a bottle of water.”)