Learn Turkish: Beklentilerin aksine sonuç çok kötü. - Contrary to expectations, the result is very bad.

Even though we translate aksine as a postposition ("contrary to"), grammatically it functions as a noun compound. Aksine is based on the noun akis (opposite/contrary). So, beklentilerin aksine literally translates to "to the opposite of the expectations". That is why the first word takes the genitive case.

What is the exact suffix breakdown of the word aksine?

The root is the noun akis (opposite). When we add the third-person possessive suffix -i ("its opposite"), the second vowel drops, making it aksi. Then we add the buffer letter -n- followed by the dative suffix -e ("to"). Together, it forms "to its opposite".

Could I use tersine instead of aksine in this sentence?

Yes, absolutely. Beklentilerin tersine means exactly the same thing. Ters (reverse) and akis (opposite) function identically in this specific grammar structure.

Can aksine be used on its own without a genitive noun before it?

Yes. When used alone, aksine acts as a sentence adverb meaning "on the contrary". For example, if you just said Aksine, sonuç çok kötü it would mean "On the contrary, the result is very bad." But to say "contrary to [something]", you must pair it with a genitive noun or pronoun.