Learn Turkish: Parayı nereye sakladıkları tam bir muamma. - Where they hid the money is a complete mystery.

In this sentence, the entire clause Parayı nereye sakladıkları (Where they hid the money) is the subject of the sentence. Subjects in Turkish take the nominative case, which means no case endings are added. You only add case markers (like the accusative in sakladıklarını biliyorum - 'I know where they hid it') when the clause is the object of another verb.

Why do we use the dative case for nereye (to where) instead of the locative nerede (at where)?

In English, you hide things in or at a place, but in Turkish, the act of hiding something is often treated as a motion towards a hiding spot. Because of this, saklamak (to hide) frequently takes the dative case to emphasize moving the object into its hidden location.

Why is the word order tam bir muamma instead of bir tam muamma?

The word tam acts as an adjective here, meaning 'complete' or 'absolute'. In Turkish, descriptive adjectives generally come before the indefinite article bir. Just like you say güzel bir gün (a beautiful day), you say tam bir muamma (a complete mystery).

Where is the main verb in this sentence?