"It is hot in the summer."
Huh? When do we use あつく and when do we use あつい, is it related with being an adverb or adjective? .. Thanks
デ is for location of an action e.g. at the restaurant "レストランで" or by means of something e.g. with chopsticks "はしで". に is for at a time e.g. at 1pm "一時に" or going to somewhere e.g. go to the restaurant "レストランに行きます" So you might be able to use the time form of に for in the summer, but not で. In this example they have used なつは instead of なつに to highlight summer as the main focus of the sentence i.e. in SUMMER I do a thing as opposed to in Winter.
When you use に as a particle, ___に is an adverbial phrase (a phrase that behaves like an adverb). なつに would be used for sentences such as "I [verb] in/during the summer", where "in/during the summer" is the adverbial phrase. なつは would be used for sentences in which you are describing the summer, such as how hot or dry it is.
I would have written 夏は暑いです so both are in kanji form, but since duolingo is atrocious at introducing and consistently using common kanji, either form should work. Report it.
the translation is gives is 'summer は hot です’?, but that means summer is hot, not in summer it is hot - shouldn't it be 夏に?
I believe either should do. 夏は indicates minor focus on the topic 夏 (が would indicate more focus, or a transition to it when the topic of conversation was something else before). に identifies it as a time during, which in a simple sentence like this is basically the same thing.