Translation:She discovered that he was unusually stupid.
If it took her some time to discover it, we can't say much about her level of intelligence...
I don't know – I think it was on their first date when he took her out for surströmming. :)
curious question, I was taught long ago that dumb is not being to know that you are so, but stupid is being aware yet deciding to make poor choice regardless. Is there a similar distinction in Swedish?
In British English we barely use dumb because of connotations with mute people. I imagine the distinction you describe may be only in American English.
Could Swedish also use "är" instead of "var" in this sentence? I.e., "... att han är ..."
Yes, definitely, though it's usually a lot more common to just use the past tense. There's no real reason for it, as far as I'm aware. German, for instance, also allows for both but heavily favours the present.
My father speaks German natively, and Swedish on a native level, well enough that almost nobody would think him anything but Swedish. This construction is one of very few that might still betray him as a non-native.
That said, if the "he" in this sentence was stupid but then somehow increased his intelligence, obviously you would need to stick to the past tense.
Whats the distinction between stupid and dumb, because dumb isn't accepted