do和forgot doing区别|forgot to

forget to do sth. 意思是忘记做某事了，这事没做，forget doing sth.意思是忘记曾经做过某事，这事做了但不记得做过了。例句： Don't forget to lock the door when you leave. 你走时别忘了锁门。

I forgot telling him about it， so I told him again.我忘了告诉过他这件事了，所以又和他说了一遍。