Learn Turkish: Keşke babasının nasihatine biraz aldırış etseymiş. - I hear he wishes he had paid a little attention to his father's advice.

The word Keşke means 'if only' or 'I wish', but the verb ending -seymiş changes who is doing the wishing. It combines the wish/conditional suffix -se with the reported past suffix -miş. Instead of expressing your own wish (which would be etseydi), you are reporting a wish or regret that you heard about. It means 'Apparently, he wishes he had...'

Why is the dative case used on nasihatine?

The verb phrase aldırış etmek (to pay attention to, to mind, to care) requires the dative case for its object. Since he is paying attention to the advice, nasihat gets the dative ending -e at the very end.

What are all the suffixes on babasının nasihatine?

It is a possessive noun phrase (his father's advice). Baba-sı-nın (father-his-of) uses the 3rd person possessive and genitive case. Nasihat-i-n-e (advice-his-buffer-to) uses the 3rd person possessive -i to link with the father, the buffer consonant -n-, and finally the dative case -e for the verb.

Is it possible to use -seymiş for the first person, like in 'Keşke etseymişim'?