【题目】我应该去接她的，但我忙于工作，把这事给忘了。
I pick her up, but I was busy with work and forgot it.

【答案】wa；upposed；to
【解析】be supposed to do sth “本来应该做某事”，但是事实上并没有发生，主语为I，谓语动词应为was。因此答案为was supposed to。