Happen to do sth.　碰巧做某事
　　普通句： I happened to pass by the Grand Theater.
　　我碰巧经过大戏院。
　　次级高手句：I happened to pass by the Grand Theater and see the poster.
　　我碰巧经过大戏院并看到海报。
　　高手句： I happened to pass by the Grand Theater and see the poster of Mariah Carey's concert.
　　我碰巧经过大戏院并看到玛利亚·凯莉演唱会的海报。
　　超级高手句： I happened to pass by the Grand Theater and see the poster of Mariah Carey's solo concert.
　　我碰巧经过大戏院并看到玛利亚·凯莉个人演唱会的海报。
　　专家句： I happened to pass by the Grand Theater and see the poster of Mariah Carey's solo concert this afternoon.
　　我今天下午碰巧经过大戏院并看到玛利亚·凯莉个人演唱会的海报。