Translation:The restaurant opens at seven in the evening.
It bothers me that "nineteen" it's not accepted. Not everybody here is from an English-speaking country. Some of us take this "Learn Italian from English" course because it's the only Italian course in Duolingo. I'm not trying to learn English, I'm trying to learn Italian, so I find this kind of situations relly annoying.
The course is about translating between Italian and English. It makes no sense to think of the norms or terms of languages other than those two. Answers should reflect the best way to translate between the two. Nothing else.
Doing a course that's specifically there to teach translation between Italian and English and complaining that the course doesn't accommodate norms from other languages is not only unreasonable, it's bizarre.
"nineteen hours" was rejected and I reported it. The 24-hour system is not only 'not wrong', it is 'correct' and used in English e.g. travel times, flights, shipping, navigation, military, formal communication. Yes, in learning Italian the translation should use the 24-hour system primarily so we embrace/accept/respect the Italian way, directly. IMO.
Well, it's something that stems from Latin. I don't know why the Romans did it (probably stems from the language which became Latin, and the one that became that, etc.), but all the other Romance languages take after it. Just be glad we don't say "undavigenti" and "duedavigenti" instead of "diciannove" and "diciotto". You should also be glad it's not like French with "sixty ten" for seventy and "four twenties" for eighty, and then "sixty twenty" for ninety. :)
alle (a + le) shows that the following noun is feminine.
alle 19 is a shortcut for alle ore 19. ore ('hours') is feminine.
If you mean 'it opens on the 19th' you'd need to say apre il 19, shortcut for apre il giorno 19 with the word giorno being masculine.
Also, 'to open at a specific hour' requires aprire alle whereas 'to open on a specific day' is aprire il.