relative clause

For me, relative clause is one of the most difficult part of grammar. There are tow point which I think is difficult for me. One is the difference between 'which' and 'that' and the other is how to distinguish the non-defining and defining relative clause.After the class and homework, I can distinguish it easily. If the relative clause been removed and the sentence can express the meaning clearly, it is the non-defining. For instance,There was only one park in this town. Someone has built over it. We used to play in the park when we were kids. So we can write a sentencs as 'The local park, where we palyed as kids, has been built over'. Because there is only one park in this town, we can understand exactly which park has been built over.