Ben was on the beach 和 Ben was at the beach 和 Ben was on the seaside 和 Ben was at the seaside 的差別在哪裡？如果不好說明，請提供一些例句。

If Ben is on the beach, then he is sitting on it, like I am sitting ON a chair, not AT a chair.

If Ben is at the beach, then he is at the location, like I am AT my house.

Seaside is another word for beach.

Hope this helps!!