Learn Turkish: Yol kapatılacağı zaman size mesaj göndereceğiz. - We will send you a message when the road is going to be closed.

The root is kapatmak (to close). We add the passive suffix -ıl to get kapatılmak (to be closed). Then we add the future participle -acak, which takes the 3rd person possessive ending -ı to link with zaman (time). Finally, the k in -acak softens to ğ due to consonant mutation, giving us kapatılacağı.

Why doesn't yol have the genitive ending (yolun), since it's the subject of the participle clause?

Grammatically, the subject of a participle clause usually takes the genitive suffix, which would make it yolun. However, when the subject is generic or indefinite (focusing on the general event of road-closing), Turkish speakers often drop the genitive. Saying yolun would sound like you are referring to a very specific, previously mentioned road.

What is the difference in meaning between saying kapatılacağı zaman and kapatılınca?

kapatılacağı zaman uses the future participle, so it means "when it is going to be closed" (implying the message will likely arrive before the closure). If you used kapatılınca or kapatıldığında, it would mean "when/as soon as the road is closed" (implying the message arrives right at or after the moment of closure).

Could I say kapatılacağında instead of ?