Learn Turkish: Hastane önünde korna çalarken polis onu gördü. - The police saw him while he was honking the horn in front of the hospital.

To say 'while doing something', you can attach -ken to either the aorist (çalar) or the present continuous (çalıyor). Both çalarken and çalıyorken are correct and mean 'while honking', but using the aorist is much more common in everyday Turkish.

I know çalmak means 'to play (an instrument)' or 'to steal'. Why is it used here for honking a horn?

In Turkish, çalmak is a versatile verb used for making sounds with many devices and instruments. You use it for playing a guitar, ringing a doorbell (zil çalmak), and honking a horn (korna çalmak).

Why are there extra letters (-ü- and -n-) in önünde?

This is a noun compound. Hastane önü means 'front of the hospital'. The -ü links 'front' (ön) to the hospital. Then, to add the locative case -de ('in' or 'at'), you need the buffer letter -n- between the vowels, giving us hastane önünde.

How do we know who is honking the horn? Could it be the police?

The verb with -ken does not take a personal ending, so the person doing the action is not explicitly marked in that word. Technically, this could mean the police saw him while the police were honking! However, context usually makes the subject clear.