"It is not hot at night."
Particles can always be omitted, especially は. I'm guessing that it's Duolingo's problem of not being able to accept different answers (yet).
The omission would make the sentence itself informal so be cautious.
Wondered the same but i think i got it: In "yoru ame ga furimasu" ,ame is the subject while in "yoru ha atsukunai desu" yoru is the subject. Not a pro at explaining but i think its safe to say when its a verb, no "ha" needed and when its an adjective "ha needed. Correct me if i am wrong.
It's not really the verbs/adjectives thing; but it is true: in よる雨がふります, よる determines the time but is not the subject of the sentence (there's 雨 for that as denoted by が). Whereas in this sentence, よる is the actual subject - for a more representative translation, think of it this way: "the night is not hot".
When you make a verb negative like this, you drop the ~い from the verb and add ~く ない
I think of it in the same way as how, in English, we drop a silent 'e' to add 'ing'.
Good explanation but you mean adjective, not verb. And it only applies to い adjectives (though I don't think duo has taught us any others yet).
A more correct translation for this would be "the night is not hot", with は being required. No need to force the は if you just want to say "it's not hot at night", I'm pretty sure it's perfectly correct without it too.
If that's so well experience a change anytime soon... Other wise I hope someone can explain