〜わけがない

1. Origins, structure, and basic meaning

• Structure: わけがない strongly rejects a possibility as implausible in light of what is known about the situation or person.
• Basic usage: 几帳面な彼が約束を忘れるわけがない。 expresses confidence that he would not forget a promise, based on his meticulous character.

2. Common pitfalls and similar expressions

• Denying possibility versus qualifying a claim
  • 分かるわけがない strongly denies the possibility of understanding. 分かるわけではない says that it cannot be claimed that someone understands and, depending on context, can be more limited than a total denial.
• Overlap with はずがない
  • 負けるわけがない and 負けるはずがない can both strongly reject defeat on the basis of evidence. Even if わけがない conveys an inability to accept the possibility, the distinction is not that one is purely emotional and the other always an objective fact.