like: Chuyện đã xảy ra khi anh ấy đang sơn phòng. → →
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It happened when he was painting his room. ☊ 
Chuyện đã xảy ra khi anh ấy đang sơn phòng. ☊ It happened when he was painting his room. ☊ Chuyện đã xảy ra khi anh ấy đang sơn phòng. ☊