"Vegetables are cheap."
How come you can omit です in some places but not here? (It marked やさいはやすい as wrong)
Desu is still needed after adjectives, its only really replaced by verbs like to go (iku)
です isnt a verb. If there is a verb, you use the ます version and it makes it polite. In the absence of a verb you use です to be polite. You can omit です without changing the meaning but you will sound rude. Ive read that だ is masculine casual です but i dont know all the stigma with actually using it.
In an audio book course I was listening to before, they explained that だ is simply the informal version of です, and didn't denote it as masculine. Perhaps it's more common for men to use the casual form? You hear it a lot in anime in any case, where they use casual form much more extensively than what is accepted in normal society.
I've read that "da" is a more masculine way of phrasing from both Nihongoshark and Japanammo if I'm recalling correctly.
だ can be used by both men and women. It's just the casual version of です。Duolingo is probably just trying to teach the formal desu and masu verbs before introducing the plain/casual forms of the verbs.
I read that you can write sentences like これはかなしい or ねこはすばらしい without です, but maybe it's slang.
You CAN, but in begining level Japanese like this teachers tend to discourage impolite Japanese
Yep, you can do that. It's not slang. Just normal spoken Japanese. Using です and ます is just the formal way of speaking.