Learn Turkish: Merkeze kaç yolcu gidiyor? - How many passengers are going to the center?

The root word is merkez, meaning center. The -e is the dative case suffix from this lesson, which translates to "to" or "towards". So, merkeze means "to the center". Because the last vowel in merkez is an "e", it takes the -e ending to match vowel harmony.

The English translation says "passengers", but yolcu doesn't have the plural suffix -lar. Why is that?

In Turkish, when you use a number or a question word asking for a quantity like kaç (how many), the noun that follows must stay singular. You never use the plural suffix after kaç, so it is simply kaç yolcu.

The dictionary form of the verb "to go" is gitmek. Why is it spelled with a "d" in gidiyor?

The root of the verb is git. When we add a suffix that starts with a vowel, like the present continuous ending -iyor, the hard consonant t softens to a d. This makes it easier to pronounce, which is a common Turkish spelling rule. So, git plus -iyor becomes gidiyor.

Since "passengers" implies multiple people, shouldn't the verb be plural like gidiyorlar?