When I was leaving home, I remembered I had forgotten my keys 和 I remembered I had forgotten my keys when I was leaving home 的差別在哪裡？如果不好說明，請提供一些例句。

The first means that you remembered when you were leaving. (so you had an opportunity to correct your mistake)
The second means that you had forgotten the keys when you were leaving and remembered it later, when you are no longer home, and can't fix the mistake)

try this:
When I was having supper I remembered I'd forgotten to take my vitamins (so I took them)
While I was at the movie, I remembered I'd forgotten to take my vitamins when I had supper. (it's too late to take them)