Sorry if this has been mentioned before, but in the one of the early lessons on casual speech, it says that だ is the dictionary/casual form of です, and that in sentences that end in です, it can be replaced with だ, or sometimes dropped altogether.
But I have seen in a few places like Reddit (I know) and Tofugu that they are apparently not exactly equivalent? That だ is strictly a copula, while です isn’t always. And forgive me if I don’t get this exactly right, but is it true that だ has the nuance of asserting what one *believes* to be true? They say because of that, you can’t ask a question with -だか instead of -ですか, since you’d be asserting a fact and asking a question at the same time, which doesn’t make sense. There’s other examples, apparently Tae Kim teaches it this way, but I won’t belabor the point.
Basically, do you guys agree with this explanation, and is it covered at some point in the lessons? If not, do you think it should be included? Or would issues from this be too unlikely to be worth mentioning?
I don’t mean to confuse anyone with this question! I’m still very new, I just happen to have seen this come up at some point ^^
So you understand how the copulas だ and です are actually connected, both come from the formal である. Particle で + verb ある "to exist" = である "exists in this way" = "to be". である got contracted to だ, and the polite form であります got contracted to です -- so だ and です are equal in everything but politeness.
The reason that there are situations where です would be used in polite speech but no copula at all is used in plain speech, is that です has a dual function of being both the copula and the politeness marker. Situations in plain speech that do not require a copula, such as after い-adjectives, may still require です in polite speech in order to convey the politeness information, instead of the grammatical information that the copula provides.
I've personally never heard the assertion that だ is somehow more certain than です, and given the above I'd doubt that's the case. 🤷♀️ If it is, it doesn't meaningfully affect how you construct sentences.
