確実性 - Degree of Certainty
普通動詞では１００％確実ですが、「かもしれない」と確実性が下がっていく時、助動詞を使って表現します。これがが使えるようになると、いちいち "I think" と言わなくても推測を表現できて便利です。
過去は 助動詞＋have＋過去分詞 で表されていることに注意してください。
| 100% sure | 現在 | He is sick. / He isn't sick. |
|---|---|---|
| 過去 | He was sick. / He was not sick. | |
| 未来 | He will be sick. / He won't be sick. | |
| 99% sure | 現在 | He couldn't (can't) be sick. |
| 過去 | He couldn't have been sick. | |
| 95% sure | 現在 |
He must be sick. He mustn't be sick. |
| 過去 |
He must have been sick. He mustn't have been sick. | |
| 未来 |
He should be sick. He shouldn't be sick. | |
| less than 50% sure |
現在 |
He may (might, can, could) be sick. He may (might) not be sick. |
| 過去 |
He may (might, could) have been sick. He may (might) not have been sick. | |
| 未来 | He may (might, could) be sick. |
MAY/MIGHT と CAN/COULD
一般的に、CAN/MAY より COULD/MIGHT の方が弱めの推量です。可能性を含んだ推量は、CAN/COULDが使われます。
Any child can grow up to be the president. 可能性を含む
Any child could grow up to be the president. can より弱め
He might be hanging around with some bad friends.
= He could be hanging around with some bad friends.
MAY/MIGHT は否定形になっても、度合いの変化はありませんが、CAN/COULD は 否定形になると「ありえない」となって、確実性が一気に上がります。
He may be upset, or he may be just tired.
He may not be upset. Perhaps he is just tired.
He could be the one who stole the money. He was there.
He couldn't be the one who stole the money. He wasn't there at that time.
MUST
MUST は論理的な言葉で、推測には何らかの根拠があります。
He must know her because they went to the same high school.
「同じ学校に通っていた」という根拠があるので、MUSTを使って推測しています。
A: Jenn must have had a car accident.
B: Why do you think so?
A: Because she has never been late. ←飛躍しすぎですが一応根拠
A: She might like him.
B: Why do you think so?
A: Just guessing. ←単なる推測
SHOULD
SHOULD は、未来形のWILL と違い、期待がこもっています。MUST 同様、何らかの根拠があります。
He should do well on the exam because he has studied very hard.
（根拠のある期待)
He will do well on the exam. （単純な未来表現)
SHOULDをif-節の中で使うと、単純動詞より確実性が下がり、「万が一〜したら。」という意味になります。
If anyone should call, please take a message. 電話のある可能性低い。
If anyone calls, please take a message. 電話のある可能性高い。条件法
