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?

Turkish is fairly flexible, but the default order is Subject–Object–(Adverbial)–Verb. Here:
• Avokadoyu (object)
• tostun üzerine (locational adverbial phrase)
• koydum (verb)
You could also say Tostun üzerine avokadoyu koydum without changing the meaning.

Do I always need to use the accusative case for objects in Turkish?

No. The accusative is only used for definite or specific objects. If the object is indefinite or non-specific, you omit it:
• Avokado tostun üzerine koydum (I put an avocado on the toast).

Why did we add a buffer consonant y in avokadoyu but not in tostun?

Buffer consonant y is needed when a suffix beginning with a vowel attaches to a stem ending in a vowel. Avokado ends in -o, and the accusative suffix is -u, so we insert y: avokado+yu. Tost ends in a consonant, so no buffer is needed for -un.