We would use the subjunctive tense when talking about an event which is uncertain, possibilities. For example: "Il est possible qu'il soit là demain" (it's possible that he will be there tomorrow) here we would use the subjunctive "soit" because it is uncertain whether or not he will be there.The conditional is used when talking about events that are uncertain, but events that would definitely occur depending on something else. For example: "Il aurait été là hier, s'il ne pleuvait pas". (He would have been there yesterday, had it not rained.) In this case, him being there was exclusively dependant on it not raining. Had it not rained, he would definitely have been there.