将主动句改为被动句时，需要注意三种不同的语态及其例句：

一般现在时的被动语态

主动句：They build a new house every year.

被动句：A new house is built every year.

一般过去时的被动语态

主动句：He wrote a letter to her.

被动句：A letter was written to her by him.

一般将来时的被动语态

主动句：They will plant ten trees this spring.

被动句：Ten trees will be planted this spring.

这些例句展示了如何将不同时间形式的主动句转换为被动语态，保持句子的语意不变，同时使句子结构更加客观。