口語課堂第22期:出其不意;當場抓獲

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Catch sb off (their) guard:趁某人毫無防備之時 to happen when sb is not prepared
The question caught him off his guard and he couldn't answer.
他對這個問題毫無準備,一時啞口.
Businesses were caught off guard by the sudden rise in interest rates.
利率突然上升使各行各業措手不及.

Catch sb red-handed:當場抓住某人(做壞事或犯罪) to find sb while they are doing sth wrong, committing a crime
The thief was caught red-handed as she was emptying the till.
小偷正準備倒錢的抽屜時被當場捉獲.