"Dobré odpoledne!"

Translation: Good afternoon!

[LynneS3]

Why is it dobré instead of Dobry? (such as dobry den and dobry vecer)

[kacenka9]

Afternoon is neutral gender. Den and večer are masculine

[PERCE_NEIGE]

So, Dobrý= masculine, Dobré = neutral, but what are the feminine and the plurals?

[matanov]

If we are speaking about greetings etc., where the case is accusative (or 4th case) –because it is shortened "Přeji Vám dobré odpoledne" = "I wish you good afternoon"– then:

feminine:
Dobrou noc = Good night

plural feminine (and masculine):
Veselé Vánoce = Merry Christmas
Veselé Velikonoce = Happy Easter

In plural neuter accusative, hard adjectives should end with á, but I dare say that standard ("correct") neuter endings are not very popular with Czechs.

I can only recommend you to have a look at declension overview (hope it won't scare you a lot):
http://mluvtecesky.net/en/grammar/adjectives
https://en.wikipedia.org/wiki/Czech_declension#Adjective

[DEcobra11]

Does odpoledne -literally- mean something like "in the afternoon". Asking because of the "od" part :)

[matanov]

Literally, it means "from-noon". The opposite is "dopoledne", which means literally "until-noon". "Poledne" is "noon".

[colondee]

Do people say "dopoledne"?

There is an English word "forenoon" which I have never heard in real life.

[endless_sleeper]

Yes, we use it. 09:00-12:00, roughly.

[demuasizigi]

Why don't need "an" after "good"? Sorry for my English...

[AgnusOinas]

You could use an article like "a" before "good", not after. But in greetings, we don't use articles at all.

Also please note that this course teaches Czech, not English.