Learn Turkish: Yangın çıkması durumunda panodaki şalteri kapatırsan iyi olur. - In case a fire breaks out, you had better turn off the switch on the panel.

This uses the formal "in case of" structure taught in this lesson: verbal noun (-mA) + possessive + durumunda. Here, we take çıkmak (to break out), make it a verbal noun (çıkma), and add the 3rd person possessive (-sı) to link it to yangın (fire). Finally, durumunda means "in the situation of". Literally, it translates to "In the situation of a fire's breaking out."

Why is the suffix -ki added to panoda?

If we just said panoda şalteri kapatırsan, it would sound like the action of turning the switch off is physically taking place on the panel. By adding -ki, we turn the location into an adjective: panodaki şalter directly describes the noun, meaning "the switch [that is located] on the panel."

Why does şalter (switch) have the accusative suffix -i?

Because it is a specific, defined object receiving the action. You aren't just turning off any switch; you are turning off the specific switch on the panel. The accusative suffix -i marks it as the definite direct object of kapatmak (to close/turn off).

What is the grammar behind kapatırsan iyi olur for "you had better turn off"?