I have said this over and over, but try to explain. you will see the difference. While Paul and others would be happy to explain, the best way is for you to find a good grammar book or search on the internet.. study the concept and post here what you learnt about ellipsis. this is how i have learnt my SC concepts. Once i tried to explain a concept, i never forgot it. It turned out that i was not correct all the time, but i never forgot that too!
SC is the most predictable part and your best friend in verbal. Spend some time thinking about a concept. there is absolutely no problem if you happen to be wrong.
I am sure Paul and others will be happy to explain, but help them by contributing your bit.