Learn Turkish: Katil hapiste kalmalıydı. - The killer should have stayed in prison.

It combines the necessity suffix -malı ("must" or "should") with the past tense suffix -dı. The buffer letter -y- links them together (kal-malı-y-dı). When you put necessity in the past tense in Turkish, it usually describes an unfulfilled obligation, translating perfectly to "should have done something" in English.

Why does the locative suffix become -te in hapiste instead of -de?

This is a quick reminder of consonant assimilation. The word hapis ends in the voiceless consonant s (part of the Fıstıkçı Şahap rule). Because of this, the locative suffix -de ("in" or "at") must match it with a hard consonant, changing its d to a t to become -te.

Is there a difference between hapis and hapishane?

Hapis refers to the state of imprisonment or confinement, though it is commonly used to mean "prison" in phrases like hapiste (in prison). Hapishane refers specifically to the physical building. Saying someone is hapiste is the most natural way to express that they are currently serving time.

Could I use lazım or gerek here instead of the -malı suffix?