How to say impossible or no reason for that to be true in Japanese

Yes, 「はずがない」 and 「わけがない」 both express strong denial or impossibility, but they have different nuances and usage contexts. Below is a detailed comparison:

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1. 「はずがない」 – “There is no way (something should be true)”

Grammar Structure

Verb (dictionary form / ない-form) + はずがない
い-adjective + はずがない
な-adjective / noun + の + はずがない

Meaning & Usage

• Expresses strong denial based on reasoning or expectation.
• It means “It is impossible” or “There is no way it should be like this.”
• It is used when something contradicts what you logically expect based on evidence, experience, or knowledge.

Examples

✅ 彼が嘘をつくはずがない。

(There is no way he would lie.) → I trust him, so I logically conclude that he wouldn’t lie.

✅ そんなに簡単にできるはずがない。

(There is no way it could be that easy.) → Based on my knowledge, it should be difficult.

✅ 彼は今、日本にいるはずだから、ここにいるはずがない。

(He should be in Japan now, so there is no way he is here.) → Logical contradiction.

🚫 Caution: 「はずがない」 is based on logical reasoning, so it should not be used for emotional or personal disbelief (see 「わけがない」 for that).

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2. 「わけがない」 – “There is no reason for that to be true”

Grammar Structure

Verb (dictionary form / ない-form) + わけがない
い-adjective + わけがない
な-adjective / noun + な + わけがない

Meaning & Usage

• Expresses strong denial, emphasizing that something is absolutely impossible or makes no sense.
• It is stronger than「はずがない」 and often conveys subjective disbelief.
• It can be used in arguments, debates, or emotional expressions.

Examples

✅ こんな難しい問題、子供に解けるわけがない！

(There is no way a child could solve such a difficult problem!) → Expressing disbelief.

✅ 彼がそんなことをするわけがない。

(There is no way he would do such a thing!) → Strong personal belief, emotional nuance.

✅ 忙しいわけじゃないけど、行きたくない。

(It’s not that I’m busy, I just don’t want to go.) → Used in a nuanced expression.

🚫 Caution: 「わけがない」 is often used when expressing personal conviction, so it can sound stronger or more emotional than 「はずがない」.

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3. Key Differences Between「はずがない」and「わけがない」

Aspect	はずがない	わけがない
Meaning	“There is no way it should be true (logically impossible)”	“There is no reason for that to be true (absolutely impossible)”
Focus	Based on logic, expectation, and reasoning	Based on personal conviction, strong denial, or emotional disbelief
Typical Use	Used when something contradicts logical expectations	Used in strong denials, arguments, and emotional disbelief
Example 1	彼が負けるはずがない。 (There is no way he would lose.) → Based on his skills, he should win.	彼が負けるわけがない！ (There is no way he would lose!) → Strong, emotional denial.
Example 2	彼は今アメリカにいるはずだから、ここにいるはずがない。 (He should be in America now, so he can’t be here.) → Based on logical contradiction.	彼がここにいるわけがない！ (There is no way he is here!) → Stronger, more emotional disbelief.

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4. When They Cannot Be Interchanged

❌「はずがない」 Cannot Replace「わけがない」

✅ こんな難しい問題、子供に解けるわけがない！ (✔️)
❌ こんな難しい問題、子供に解けるはずがない！ (⛔ Sounds unnatural)

• This is an emotional rejection, so 「わけがない」 is better.

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❌「わけがない」 Cannot Replace「はずがない」

✅ 彼は今、日本にいるはずだから、ここにいるはずがない。 (✔️)
❌ 彼は今、日本にいるわけだから、ここにいるわけがない。 (⛔ Sounds unnatural)

• This is a logical contradiction, so 「はずがない」 is better.

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5. Summary

Expression	Meaning	Stronger or Weaker?	Logical or Emotional?	Common Context
はずがない	“There is no way it should be true”	Weaker	Logical reasoning	Based on evidence, facts, or expectations
わけがない	“There is no reason for that to be true”	Stronger	Emotional conviction	Used in strong denials or arguments

💡 If you want to logically deny something, use 「はずがない」.
💡 If you want to strongly and emotionally reject something, use 「わけがない」.