happen是不及物動詞，沒有被動語態，不可以接賓語，也就不存在把賓語提前變成被動語態的了。what happens to trees in differen可以是what happens to trees in differen seasons。意思是樹木在不同的季節會發生什麼變化。happen的用法常見的有如下幾種情況：
1. 表示“某地(某時)發生了什麼事”，常用“sth.+ happen +地點/時間”這一結構來表達，此時主語應是事情。例如： The story happened in 2003. 這個故事發生在2003年。 An accident happened in that street. 那條街上發生了一起事故。
2. 表示“某人出了某事(常指不好的事)”，要用“sth.+ happen+to sb.”這一結構來表達。例如： A car accident happened to her this morning. 今天上午她發生了交通事故。 What happened to you? 你怎麼啦?
3. 表示“某人碰巧做某事”，要用“sb.+ happen+ to do sth.”這一結構來表達。例如： I happened to meet a friend of mine in the street yesterday. 昨天我碰巧在街上遇到了我的一個朋友。
4. happen表示“碰巧或恰巧發生某事”時，還可用“It happens / happened that...”這一結構來表達。例如： It happened that Brian and Peter were at home that day. 碰巧那天布萊恩和彼得在家了。 It happened that he had to take part in a meeting that afternoon. 碰巧那天下午他不得不參加一個會議。 注：that從句中的主語是人時，此種結構可以與“sb.+ happen + to do sth.”結構互換。例如： It happened that Brian and Peter were at home that day. = Brian and Peter happened to be at home that day.
happen是不及物動詞，沒有被動語態，不可以接賓語，也就不存在把賓語提前變成被動語態的了。what happens to trees in differen可以是what happens to trees in differen seasons。意思是樹木在不同的季節會發生什麼變化。happen的用法常見的有如下幾種情況：
1. 表示“某地(某時)發生了什麼事”，常用“sth.+ happen +地點/時間”這一結構來表達，此時主語應是事情。例如： The story happened in 2003. 這個故事發生在2003年。 An accident happened in that street. 那條街上發生了一起事故。
2. 表示“某人出了某事(常指不好的事)”，要用“sth.+ happen+to sb.”這一結構來表達。例如： A car accident happened to her this morning. 今天上午她發生了交通事故。 What happened to you? 你怎麼啦?
3. 表示“某人碰巧做某事”，要用“sb.+ happen+ to do sth.”這一結構來表達。例如： I happened to meet a friend of mine in the street yesterday. 昨天我碰巧在街上遇到了我的一個朋友。
4. happen表示“碰巧或恰巧發生某事”時，還可用“It happens / happened that...”這一結構來表達。例如： It happened that Brian and Peter were at home that day. 碰巧那天布萊恩和彼得在家了。 It happened that he had to take part in a meeting that afternoon. 碰巧那天下午他不得不參加一個會議。 注：that從句中的主語是人時，此種結構可以與“sb.+ happen + to do sth.”結構互換。例如： It happened that Brian and Peter were at home that day. = Brian and Peter happened to be at home that day.