Usually reading the FAQ in the rulebook (or the wiki) can sort these questions out.
However, I will admit that I have also gotten rules like this wrong in the past, although neither of these examples were solved by Dominion online. Rather, one was solved by buying another expansion, and the other by looking at the wiki.
Until I got Seaside and figured out that Durations are a thing, I thought Bridge acted like a Duration, making cards cost $1 less this round, instead of this turn. I got confused because of the wording distinction with Quarry (I got Prosperity before Seaside because it was recommended first), which said 'while this is in play'. I didn't realize the distinction was to prevent it from stacking when getting Throned, if such a card eventually came out for Treasures (which it did). Not even sure why I thought that; it wouldn't be good enough for $4, but I still bought it anyway because it seemed cool.
Also when I first got Adventures, I would always put out Peasant and Hireling because I loved those cards, and whenever I got Disciple, I would Disciple the Hireling, because it was a free extra Hireling play and extra Hireling card, without realizing the Disciple was supposed to stay out with the Hireling. WAY less overpowered that way, as Disciple takes a while to get.
Looking back, both of these could probably have been solved by looking at the FAQ, and both of the correct ways to play these cards made tracking a lot easier as well.