forget doing sth.的意思是“忘记做过某事（事情已做）”，表示已经做那件事，只是忘记了曾经做过。

I forgot borrowing a book from you. 我忘记曾经向你借过书这件事。

I just forgot fetching the file. 我忘了已经拿了那份文件了。

I"ll never forget finding that rare old coin in my gardon. 我永远不会忘记在我花园里找到的那枚稀罕的古币。

Mary will never forget seeing the exciting film. 玛丽永远不会忘记看了那部激动人心的电影。

I forgot writing to him,so I wrote again. 我忘了给他写过信，所以我又写了一封信。

He forgot turning the light off. 他不记得他关了灯了

I forgot writing a letter to him. 我忘了给他写过信。

I forgoting closing the window! 我忘记我已关了窗。