Learn Turkish: Sonucu tahmin edemedim. - I could not guess the result.

Sonuç takes the accusative case ending -u because it is the specific direct object of the verb. When you add a vowel suffix to a word ending in ç, the consonant softens to c. That is why sonuç becomes sonucu.

I know tahmin etmek means to guess, but why does et change to ed in edemedim?

Just like with sonuç, the t at the end of the helper verb etmek softens to a d because it is immediately followed by a vowel suffix (the -e- in this case).

What is the exact difference between tahmin etmedim and tahmin edemedim?

Tahmin etmedim simply means "I didn't guess." The extra -e- in tahmin edemedim is the negative ability marker, which changes the meaning to "I could not guess."

Where did the "could" (ability) suffix -ebil go in the negative verb edemedim?