Learn Turkish: Kıyafetleri hazırlıyorum, çantada her şey var. - I am preparing the clothes, everything is in the bag.

Even though "everything" is one word in English, in Turkish her (every) and şey (thing) are always written separately. It is a common spelling mistake for beginners to write them together, but her şey is the only correct way.

Why does kıyafetleri end with the -i suffix here?

The base word is kıyafet (clothing) and kıyafetler is the plural (clothes). We add the specific direct object ending (the accusative case -i) because you are preparing the specific clothes, not just clothes in general.

The verb for 'to prepare' is hazırlamak. Why does it become hazırlıyorum and not hazırlayorum?

When a verb stem ends in -a or -e (like hazırla-), adding the present continuous -yor suffix causes that final vowel to drop or "narrow". Here, the -a changes to -ı to match vowel harmony, resulting in hazırlıyorum.

Since her şey means "everything", does it act as a plural subject?

No, it acts as a singular subject! Even though "everything" refers to many items, the word şey is grammatically singular. That is why we simply use var (there is) instead of adding any plural endings.