"Good evening, thank you."
Translation:Dobrý večer, děkuji.
I cannot differenciate Dobry, dobre and dobrou. when to use one or the other! :(
You need to learn the declension table for the word dobrý. You can find it in https://en.m.wiktionary.org/wiki/dobrý
dobrý is nominative masculine singular case, among others. Dobré is nominative neuter singular case, among others. Dobrou is instrumental feminine singular case, among others. Too numerous to list here.
what are the different versions of dobry (good) and when do you use them?
I believe you are talking about declension. In the singular form and the first grammatical case (nominative), there are three gendered 'versions' of the word.
Masculine: "dobrý" -ý Feminine: "dobrá" -á Neuter: "dobré" -é
Note that the gender always refers to the grammatical genred of the noun the adjective refers to, which may be unrelated to the actual gender of the object.