Questions & Answers about Tämä hyvä teko teki päivän erityiseksi.
Why does the word for "special" have the -ksi ending here (erityiseksi)?
This is the translative case, which expresses a change of state. When you use the verb tehdä (to make) to say that you "make something [adjective]", Finnish requires that adjective to be in the translative case. The deed changed the state of the day, making it become special.
What is the base form of erityiseksi, and how did it change to get that ending?
The base form is the adjective erityinen (special). In Finnish, words ending in -nen drop the -nen and replace it with -se- before adding most case endings. So, erityinen becomes the stem erityise-, and then we add the translative ending -ksi.
Could I just use the basic form and say "Tämä hyvä teko teki päivän erityinen"?
No, that would sound wrong in Finnish. While English uses the basic adjective form ("made the day special"), Finnish strictly requires the translative case (erityiseksi) as the object complement to show the result of the verb tehdä.
Why is the object päivän in the genitive form instead of the partitive?
It is in the genitive-looking accusative case. Because the action of making the day special is viewed as a complete, finished action, the total object päivä takes the -n ending. If the action were somehow incomplete or ongoing, you would use the partitive form päivää.