忘记曾经做过某事用英文怎么说

"Forget doing something" 可以用英文表达为 "forget about doing something" 或 "forget to have done something".

例如：

I forget about calpng my grandma yesterday. (我忘记昨天打给我奶奶的电话了。)
I can't bepeve I forgot to have done my homework last night. (我不敢相信我昨晚竟然忘记了完成我的作业。)

这两个说法都是常用的表达方式。