"If you tell me, that any person is in love, i easily understand your meaning, and form a just conception of his situation; but never can mistake that conception for the real disorders and agitations of the passion."
I believe Hume is trying to say that unless he is experiencing something, he cannot know what it feels like. You can easily understand waht it is like to be inlove, afraid, sad, happy, etc. but unless you are feeling those emotions, you cannot conceive their situation. I often try to put myself in other people's positions to understand where they are coming from or to treat them how I would want to be treated, but it is harder than you think. I hear myself saying "oh I would be so mad" or "I would hate that", but how do I know that unless I am in that situation in that instance?