How could I not know this rule. Castle rights

I don't consider myself an expert chess player (but hoping to get there in the next 5-10 years) but surely not a beginner. I just started playing some tournaments and realized I don't even know one of the most basic rules of chess.

Castle rights. Of course if a rook or queen cuts off the link between your king and rook you can't castle through that. But sometimes the computer will let you castle through a break in that link. Is it only if the force of the piece does not cut that link in two. In other words, if the rook of the opposition "hits" one of the pieces but does not break the link you still have castleing rights? I swear sometimes my bishop cuts my opponents connection between rook & king in two and the computer lets him castle. 

Can't believe I don't know this but I've really only played online chess and want to start playing more tournaments and I don't know this basic rule. I guess I could google it but if anyone has any thoughts please let me know.