Grammar Info
N4 Lesson 8: 13/18

筈(はず)がない
hardly possible, cannot be, highly unlikely, improbable

Structure
Casual
Polite
Verb + はず + が + ない
[い]Adjective + はず + が + ない
[な]Adjective + + はず + が + ない
Noun + + はず + が + ない
Verb + はず + が + ありません
[い]Adjective + はず + が + ありません
[な]Adjective + + はず + が + ありません
Noun + + はず + が + ありません
Details
Register Standard
使用域 一般
Information
Like はず, coming from the kanji 筈(はず), which suggests that something is ‘bound’ to be true, はずがない suggests that something is ‘not bound’ to be true. This expression is usually translated as ‘hardly possible’, ‘improbable’, or ‘unlikely’, but in reality, it is a bit weaker than these, and translates more literally to ‘(A) is not the only possibility’.
As はずがない (a combination of 筈(はず), the case-marking particle , and the い-Adjective ない) is a phrase that starts with a noun, it may be used after any word that could usually connect with a noun.
  • 1時間(じかん)早(はや)くに家(いえ)出(で)から遅(おく)れはずがない
    Because I left the house an hour early, it is improbable that I will be late. (That I am bound to be late is not true)
  • この箱(はこ)重(おも)はずがない
    It is hardly possible that this box is heavy. (It is not the case that this box is bound to be heavy)
  • サリー家(いえ)立派(りっぱ)はずがない
    It is highly unlikely that Sally's house is splendid. (That Sally’s house is sure to be splendid is not the case)
  • あそこ人(ひと)トムはずがない
    It is highly unlikely that the person over there is Tom.
Caution
There is a big difference in nuance between ないはず ‘bound not to’, and はずがない ‘not bound to’. ないはず strongly negates the (A) statement, and expresses the speaker’s opinion that it is almost definitely not true. はずがない on the other hand, simply suggests that (A) being almost 100% true is not the case.
  • 彼(かれ)車(くるま)高(たか)くないはず
    His car would not be expensive. (His car is bound to not be that expensive)
  • 彼(かれ)車(くるま)高(たか)はずがない
    It is hardly possible that his car is expensive. (That his car is bound to be expensive is not true)
Slow Male
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夏(なつ)雪(ゆき)降(ふ)るはずがない
It is hardly possible for it to snow in the summer.
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あのビル古(ふる)すぎなので病院(びょういん)はずがない
Because that building is so old, it is hardly possible that it is a hospital.
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有名人(ゆうめいじん)人気(にんき)から寂(さび)しいはずがない
Famous people are popular so it seems hardly possible for them to be lonely.
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Books
Genki II 2nd Edition
Page 172
Tae Kim's Japanese Grammar Guide
Page 306