That would be just the informal way, yes - dropping the second part of quite a lot of phrases makes it informal - arigatou gozaimasu >> arigatou, onegaishimasu >> onegai, etc
Uhhh... "Oyasumi" means vacation, rest or break from something. However, this different than when people say "Oyasuminasai." They're using Base2 command form in this situation. Yasumu = to rest. Yasumu>B2=yasumi. Add an honorific "O" and 'nasai.' O-yasumi-nasai. In other sitiuations, they would just say "oyasumi" when referring to a break from school, work, normal routine, etc. It's not the informal way of saying "oyasuminasai." If you said oyasumi to a Japanese peraon, they'd be waiting for you to talk about a rest or break of some sort.
Correction on my part: I wrote too quickly. It is used as an informal way of saying good night, but it also is very often used as a break. Every time I'd heard it, it only was talking about a rest of sorts. Sorry for the confusion.
Why can't I say "こんばんは" ?
In English, "good evening" is implied as a greeting or a farewell. "Good night" can sometimes be synonymous with "good evening" but it can also mean that you (or the person you're talking to) is headed to sleep.
Because of its dual meaning, Duolingo generally uses "good night" when referrering to sleep (oyasuminasai) and "good evening" when referring to the greeting (konbanwa).
But it's also used when you're leaving a gathering, say a party or a dinner gathering, and it's late.
It's kind of implied that you'll be going to bed when you get home,but not necessarily expected. It does signal that you're not expected to be heard from again that night.
It's more like "good bye" for tonight, if you're not at home (or you are at home saying it to guests who are leaving).
But if you're at home and saying it to family members/house mates, (or guests that are not leaving) then it means you're going to bed now.
Anyone got any tips on prouncing this correctly, i keep saying something more along the lines of おやすみまさい and its really hard to say it right while speaking quuckly.
If there are multiple interpretations of an english phrase, Duolingo needs to add hints about context. Especially for cultural phrases, knowing the context and meaning is more important than a direct set of english words that one might have said in many different contexts.