Learn Turkish: Hastanede kim var? - Who is in the hospital?

The locative suffix (meaning "in/on/at") changes to match the noun. Because the last vowel in hastane (hospital) is a front vowel (e), vowel harmony rules tell us to use the e version of the suffix. Also, because the word doesn't end in a hard consonant (like p, ç, t, k), the suffix starts with d instead of t. So, we get hastanede.

Hastane ends in a vowel. Don't we need a buffer letter like y before adding the suffix?

No buffer letter is needed here! The buffer letter y is used to separate two vowels when a suffix starts with a vowel (like the "to/towards" suffix -a or -e). Since the locative suffix -de starts with a consonant (d), you can attach it directly to the word without any extra letters.

Why does this sentence use var? Doesn't var mean "have"?

var literally translates to "there is" or "exists." While it is used in Turkish to express "having" something (for example, "my car exists" means "I have a car"), on its own it just means that something or someone is there. So Hastanede kim var? literally means "In the hospital, who exists?"

Why is the question word kim (who) placed in the middle of the sentence?