Why is 'be at' incorrect for 'cxeesti'?
As in English, being at an even is not quite the same as attending it:
Would "be present at" work?
ne povi unable?
How is this different that "Adamo cannot attend..."?
Because it is the past tense "povis" not the hypothetical "povus." Basically, the sentence is saying that the conference already happened and Adamo couldn't make it. Not that, if it were to happen, he wouldn't be able to make it.