"Thank you and good night!"
Translation:Děkuji a dobrou noc!
The adjective "dobrý" ends with ý/á/é in the nominative singular, so it belongs to a group of "hard" adjectives and is declined according to pattern "mladý". To find the right ending for the adjective, look at the noun that follows after the adjective (this time it is "noc", which is a feminine noun). You know that among endings for pattern "mladý", you have to look for the feminine ones. And since the phrase "dobrou noc" is a wish, both the noun and the adjective (dobrá noc) have to be in accusative -> dobrou noc.
Jamie08MD, your comment is so useful because at this part of the Czech course, we have not been taught about accusative endings. And, I would not have guessed that "night" in "Good night" would be in the accusative case. Without your explanation, I would be left wondering what kind of strange ending is "ou" in "dobrou." Many thanks!