Learn Turkish: Kamyon ile otobüs çarpışır çarpışmaz siren seslerini duyduk. - We heard the siren sounds as soon as the truck and the bus collided.

This structure uses the aorist tense to mean "as soon as." You simply take the positive aorist of a verb (çarpışır) and immediately follow it with the negative aorist of the same verb (çarpışmaz). Notice that it doesn't take any personal endings; the actors (the truck and the bus) are stated right before it.

What is the difference between çarpmak and çarpışmak?

Çarpmak means "to hit" or "to strike." The -ış in çarpışmak is the reciprocal suffix, which shows that a mutual action is happening between two or more subjects. So, çarpışmak specifically means "to hit each other" or "to collide."

Can you break down the suffixes on seslerini?

It comes from the noun compound siren sesi (siren sound). First, the plural suffix makes it siren sesleri (siren sounds). Because we are talking about specific sounds that we heard, it needs the accusative case. We add the buffer letter -n- and the accusative suffix -i to get siren seslerini.

Why use ile instead of ve to say "truck and bus"?

You could use ve, but ile (meaning "with" or "and") is extremely common for linking two nouns in a pair. It feels very natural here because the truck and bus are participating in a joint action (colliding with each other).