I'm beginning to sympathise a little with Corbyn's repeated reluctance to answer hypothetical questions, although perhaps he could still do with explaining why he doesn't, just in case I am being too charitable.
Still, the way I see it, if -- God forbid -- I were leading the country from June 9th, is that I would go into the negotiations fully intending to, and working towards, leaving the EU, and fully expecting to leave it within five years (probbaly sooner). *But* -- and here, I think, is where Corbyn's coming unstuck -- there may be circumstances in which leaving becomes obviously bad for the country, and so I wouldn't want to commit to leaving 100%, come what may. I can't describe those circumstances, except for the grotesque and obviously unrealistic ones, but it stands to reason that such circumstances exist. So committing to leaving unconditionally is just not something I'd want to do. The point is that it wouldn't detract from my intentions or approach, or at least I'd try not to let it, and the other point is that such circumstances as might arise change the probability of leaving from 100% to about 99.998% or something.
Even this may seem heretical or undemocratic to some people, but really it's just stating an obvious hypothetical: if it becomes clear that leaving the EU is not remotely in the best interests of the UK, then the government should be prepared to change course accordingly. But the course -- of leaving -- is still laid in, still to be followed, and the "if" is a really, really big if that I don't expect to happen, at least not enough to change that course. It's just about keeping options open and not showing our hand.
Whether or not Corbyn meant this by refusing I don't know -- although for those who doubt him, it's worth pointing out that he was for a long time in favour of leaving the EU, whereas May was (at least somewhat) pro-Remain. Possibly still is in favour of leaving, for all I know.