把主动语态改为被动语态非常简单，可以遵循以下几个步骤：

1. 先找出谓语动词；

2. 再找出谓语动词后的宾语；

3. 把宾语用作被动语态中的主语；

4. 注意人称、时态和数的变化。

例：

1. Bruce writes a letter every week. →A letter is written by Bruce every week. 布鲁斯每星期写一封信。

2. Li Lei mended the broken bike this morning.→The broken bike was mended by Li Lei this morning. 李雷今天上午在修理坏的自行车。

3. He has written two novels so far.→Two novels have been written by him so far. 至今他已写了两部小说。

4. They will plant ten trees tomorrow.→Ten trees will be planted by them tomorrow. 他们明天将要种植十棵树。

5. Lucy is writing a letter now.→A letter is being written by Lucy now. 露西正在写信。

6. You must lock the door when you leave.→the door must be locked when you leave. 当你离开的时候你必须把门锁上。