What does 'I do.' mean here?

[diamondypy]

Hi, everyone. I read a dialogue like this:
A: I hope he hasn't failed his exam.
B: I do.
What does 'I do.' mean here?
THANKS.

[Tukanja]

I have no idea what "I do" could mean.
"So do I" would work for me.

[Joe]

In normal idiomatic English "I do" in this context could mean: I do hope he HAS failed his exam.

The exact meaning might be coloured by the stress and intonation. In your example, there might be stress on the "I" to emphasize the difference of opinion.

Tukanja's "So do I" is certainly a possible to response to A, but it would mean that B agrees with A. In fact, I suspect, B specifically does not agree with A. As indicated, it's difficult to be certain without hearing the dialogue.

[faizanijaz]

Thank you very much Josef.
Are you able to tell me why I learn wrong english?
Or aren't you?