这要看改成被动语态后的主语情况，谓语动词（或者助动词加动词原形）要随着主语的变化而变化，这是英语语法当中很重要的“主谓一致”原则。 比如：My hometown has experienced great changes.改成被动语态之后变为：Great changes have taken place in my hometown.
