Learn Turkish: Kral hazineyi mağaraya saklamış olsa gerek. - The king must have hidden the treasure in the cave.

It combines the verb root sakla- with the evidential past suffix -mIş and the phrase olsa gerek (it must be). This specific structure is used to make a strong logical deduction about a past event, translating perfectly to the English 'must have done something'.

Why do we use the dative case mağaraya (to the cave) instead of the locative mağarada (in the cave) for where it was hidden?

In Turkish, the verb saklamak (to hide) requires the dative case (-yA) for the hiding place. Turkish thinks of hiding as a directional action moving an object into a place (hiding it 'into' the cave), whereas English focuses on the static location (hiding it 'in' the cave).

What is the difference between saklamış olsa gerek and saklamalıydı?

They express completely different ideas. saklamış olsa gerek is a logical deduction meaning 'I assume he hid it based on evidence.' saklamalıydı combines the necessity suffix -mAlI with the past copula, meaning 'he should have hidden it' (but he did not).

Why does hazineyi have the -yi suffix here?