Once you understand the difference between a main success scenario, an alternate scenario and an exception scenario, you may wonder why it took you so long to get there. The reason probably being that the explanation was pretty abstract, talking about scenario's that do or don't have the same goal, post-conditions that add to or are instead of those of the main success scenario, blah, blah, blah...
Having gone this road myself, I started to wonder if there is a simpler way to explains what should be pretty natural for most of us. And as always, finding an example that relates to day-to-day life did the trick for me to explain it to others. The example being the following.
Suppose that have to drive from A to B. Your goal being to be in time for a customer meeting. The post-condition being that you successfully reached your destination in time.
So the main success scenario is that you drive from A to B, without interruptions.
An alternate scenario would be that you have to take a small detour by going through a gas station to get some gas. An extra post-condition this detour might add to the one of the main success scenario, could be that you have to obtain the gas bill, otherwise you employer won't cover your expenses.
An exception scenario might be that your car breaks down, and you will never be in time for the customer meeting. A post-condition of this exception scenario might be that you have to inform you customer that you have to cancel the meeting. This post-condition replaces the post-condition of the main success scenario.
Other exception scenario's might concerning a serious traffic jam, getting busted for speeding, car-jacking, and so on.
Now that was not too difficult, was it?