"It is warm today, and it is cool tomorrow."
I had that problem too. Turns out the problem was that I was adding the word でも into the space of the comma. When I removed it, duolingo accepted it. However, I think that maybe it still should be accepted, as でも can mean "also" as well as "but", both of which seem applicable. Can I get someone to confirm/deny this?
@Guillermo2436 if you are still lost over there, it might be helpful to relate the usage of this ～く to what you see when you negate i-adjectives. You are basically using a clause to modify the next one「『今日は暖かく』→『明日は涼しいです』】this allows you to create a continuous sentence, similar to how you would connect 暖かく→ない in order to negate the concept of being described as 暖かい。
I can't find this anywhere except a single quora answer which says that it gives distinction between the two clauses more so than standard て-form. I've not seen this form anywhere else and it seems almost like a mistake. Can anyone clarify better? https://www.quora.com/Japanese-language-Why-use-the-ku-form-instead-of-the-te-form-when-connecting-a-sentence-to-another-one?share=1
This lesson literally introduces using く for adverbs. Why on earth would they use this form in the place of a standard て-form adjective?
There are many ways to say "and" in Japanese (i.e. to use a conjunction). そして、と、し、も、てーform (verbs and い-adjectives), で (nouns and な-adjectives), and others. It all depends on the sentence and what is being joined. Some are their own words or particles; others are conjugations of nouns, adjectives, etc. There is not single equivalent to the English "and."