Learn Hardcore Turkish: Avokadoyu tostun üzerine koydum.

It’s the accusative case marker -yU (with a buffer y because the noun ends in a vowel). The buffer consonant y prevents two vowels clashing. The accusative is used because avokado is a definite, specific object in the sentence (“the avocado”).

Why is tostun in the form tostun with -un?

This is the genitive (possessive) case -un, required by the postposition üzerine (“on top of”). When a noun is governed by certain postpositions, it must be in the genitive and often carry a possessive agreement (here tost+-un indicates “of the toast”).

What is the role of üzerine?

Üzerine is a postposition meaning “on/onto (something).” It follows a noun in the genitive case and shows location or direction onto something.

Can I say tostun üstüne instead of tostun üzerine? Are they the same?

Yes, practically they are synonyms. Üzerine and üstüne both mean “on top of,” but üzerine is slightly more formal. Both require the noun in the genitive case.

Why is avokadoyu placed before tostun üzerine instead of after?