However, I have seen people at JLPT N3 level and beyond getting into trouble with more complex sentences simply because they have never learned what da/desu really does.
As so often, the textbook explanations don’t make it properly clear.
Actually some of them do tell you, in passing, the technical term that would give you the key to the mystery.
But then they don’t explain that term properly and carry on with the usual rough-and-ready dumbed-down explanations that leave your Japanese understanding like a poorly-built building that will come down when a strong wind blows.
So is this going to be some complicated technical explanation that the textbooks don’t tell you because it’s too difficult?
Not at all! It’s very simple and straightforward. In fact, it’s easier than what they do tell you. And once you know it your understanding of the da/desu function will be as solid as a rock.
This is part of a mini-series or “story arc” (heh heh) within the current sequence of videos, because part of the confusion is intertwined with the way i- and na-adjectives aren’t properly explained (one of my earliest grammar articles which I made into a video last week) and will lead on to a discussion of “na no desu” and related constructions in videos to come.
So if you need more desu (and who doesn’t?) watch this video now.
For more information on the concepts in this video, please see:
I and Na Adjectives: what the textbooks don’t tell you (article and video)
One of the problems with textbook Japanese is that they treat Japanese grammar as if it were a series of random, unconnected “points” when really it is an organic, beautiful and amazingly logical whole (much more so than European grammars, including English).
In fact, I think it is because they expect language to be complicated and random, like European languages, that they treat Japanese as if it were so, and thus make it so for the poor learner.
Da/desu fits together with everything else in basic Japanese to make a rounded whole.