✔ 最佳答案
當increase 和 decrease 這兩個詞語被用作 transitive verbs 的時候, 就能夠使用在被動句式中。
例子:
Active: His boss finally increases(decreases) his salary.
Passive: His salary is finally increased(decreased) by his boss
當increase 和 decrease 這兩個詞語被用作 intransitive verbs 的時候, 就不能夠使用在被動句式中。
例子:
Active: The population of Hong Kong has increased steadily.
Passive: 因為這句的 increased 是 intransitive verb，所以不能轉 passive。
你中學英文老師用的是 intransitive verb 例子，例如： his weight has increased/decreased，所以這兩個詞語不能使用在被動句式中。
但是，如果用 transitive verb 的例子，例如：Peter increases/decreases the weight of his model car by adding metal wheels to it，就可以轉 passive:
The weight of his model car is increased/decreased by Peter by adding metal wheels to it.
2009-06-19 07:49:36 補充：
His salary has been increased (v.t.) 和 His salary has increased (v.i.) 意味上 (implication) 有分別。因為第一句是省畧了加他人工的人(他的老板)，即意味着他做工做得好，他老板特意加他人工。第二句只是說他加了人工，可能是每年慣性調薪，沒有意味有人特意加他人工。
