forgot to turn off the light 與He forgot turning off the ligh

題目：

forgot to turn off the light 與He forgot turning off the light 兩句的區別.
意思我知道、麻煩幫我解釋一下。

解答：

He forgot to turn off the light.他忘了關燈了（根本沒關）.
He forgot turning off the light.他不記得他關了燈了（跑回家又關去了）.