Lesson 53 Must have

ポイント
In the present tense, we use the 法助動詞 “must” for necessary actions. But what about its past tense form, “must have + 過去分詞”?

ダイアログ
Jenny: Hi Kyoko. Where were you last week? I didn’t see you in the office.
Kyoko: My daughter had a cold last week and couldn’t go to school, so I must have stayed at home to look after her.
Jenny: I’m confused. You must have stayed at home?

解説
Why is Jenny confused by Kyoko staying at home to look after her sick daughter? Well, Kyoko meant to say “家で過ごさなければならなかった,” but she actually said　“家で過ごしたかもしれない.” Why? “Must” has two meanings: one for necessary actions and one for things that are most likely true. For example, “He must have this book” can mean either “He needs to have this book” or “He most likely has this book.” However, the past form “must have” only has one meaning: things that were most likely true. We use “needed to” and “had to” for necessary past actions. So Kyoko should have said, “I needed to stay at home” or “I had to stay at home.”