"I go to bed at eleven."
Translation:Vado a letto alle undici.
Nowadays, sometimes, we use the 12 hour clock too. I think duolingo tells that to say ventitrè anziché (sorry, idk how to translate) undici is wrong, because it'basically wrong: you cannot say OH, ELEVEN IS THE SAME AS TWENTYTHREE, BC I'M ME AND EVERYTHING I SAY IS RIGHT . Understood?
You don’t shorten “le” in front of a vowel, so you also can’t contract “alle.” It’s an articulated preposition (preposition + article, in this case a + le), so it follows the rules for the article. So:
l’una (because it’s “la,” singular to go with “una”) / all’una
Le undici / alle undici