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Imagine the case where B says 'now I know how to go on' but then hesitates and cannot do it. It is possible that B was wrong to say 'now I know how to go on' and it's also possible that he was right but now is not able to go on. - This is a grammatical point being made by Wittgenstein. With understanding it is possible that you can think you understand but you don't (unlike being in pain - you cannot think you're in pain and not be in pain) it is also possible that you can understand something but that you are then not able to satisfy one or another behavioural criterion of understanding.

It is possible, in the situation described above, that B realises that he made a mistake in thinking that he could go on. It is also possible that B was able to go on but was distracted or something like that. Wittgenstein says 'we shall say different things in different cases'.

Thinking about cases you could imagine that someone says 'ah! Now I know how to go on!' but then soon afterwards says 'hang on a minute. No. That's not right'. In which case we would suppose that the person had thought they'd understood but hadn't (I suppose it is also the case that someone might say 'no that's not right' when in fact what they'd thought was right and they had understoofd..