Learn Turkish: Yarın sabah havalimanına gidiyoruz. - Tomorrow morning we're going to the airport.

This is due to a rule called consonant mutation. When certain words end in t (like the verb root git-) and take a suffix starting with a vowel (like the present continuous -iyor), the hard t softens to a d. This makes it easier to pronounce!

Why is the "to" suffix attached as -na in havalimanına? Shouldn't the buffer letter be "y"?

Normally, yes! But havalimanı (airport) is a compound noun made of hava (air) and liman (port) plus the 3rd person possessive suffix -ı. There is a special rule in Turkish: whenever you add a direction/location suffix (like the "to" suffix -a) to a word that ends in a 3rd person possessive suffix, you must use the buffer letter -n- instead of -y-.

The sentence uses yarın sabah (tomorrow morning), but the verb gidiyoruz is in the present continuous tense. Is that correct?

Yes! Just like in English ("we are going tomorrow"), Turkish frequently uses the present continuous tense (-iyor) to talk about fixed, planned events in the near future. The time phrase yarın sabah makes it clear that the action is happening tomorrow, not right now.

Why does sabah (morning) not have any suffixes attached to it here?