I don't understand, but I think that saying da has no subject is more confusing, because it does.I don't think so. I think if I had said that the subject was an implied tu, it might be misinterpreted (i.e. that "implied tu" means das rather than tu das, and tu (nom.) da doesn't make any sense). But then again, das doesn't bear the same force as da (and there's an "s" on the end).