疑う

1. Origins, structure, and basic meaning

• Structure: 疑う is a transitive godan verb read うたがう and means to doubt a fact, suspect a person, or find something hard to believe.
• Basic usage: Mark a suspected person in 友達を疑う, and use the idiom 耳を疑う for news that seems unbelievable.

2. Common pitfalls and similar expressions

• を marking a suspected person
  • In 証拠もないのに友達を疑う, を marks the friend being suspected. もないのに adds the critical condition that this happens despite there being no evidence.
• 耳を疑う
  • This does not literally mean testing one's hearing. It is an idiom for finding what one has heard too unexpected to believe.
• 疑う and 信じない
  • 信じない states a lack of belief, while 疑う focuses on questioning whether something is true or suspecting someone's conduct.