Learn Turkish: Bu karanlık bodrumda gizli bir oda mı var? - Is there a hidden room in this dark basement?

In Turkish, the indefinite article bir (a/an) almost always goes between the adjective and the noun. If you put it first and said bir gizli oda, it would sound like you are counting exactly 'one hidden room' rather than just saying 'a hidden room.'

We usually see var mı (is there?) at the end of a question. Why are the words separated as mı var here?

The question particle mı places emphasis on the word right before it. While gizli bir oda var mı? is a perfectly correct and neutral way to ask 'is there a hidden room?', placing mı right after oda shifts the focus slightly to emphasize the room itself: 'Is it a hidden room that is down there?'

How is the adjective gizli (hidden) formed?

It is built from the noun giz (secret or mystery) combined with the common suffix -li, which means 'with' or 'having.' So gizli literally means 'having a secret,' which is used as the adjective for 'hidden' or 'secret.'

Which grammar rule gives us the 'in' meaning for bodrumda?

That is the locative case suffix, which is used to show location (in, on, or at). The base word is bodrum (basement). Because its last vowel is 'u' (a back vowel), vowel harmony rules dictate that we use -da instead of -de to create 'in the basement.'