Question

当一个句子用被动语态时，动词用过去分词，什么时候要加be动词？什么时候不要？

Answer 1

如果这个被动短语做主语时要加be,并且要是ing形式，如果做状语或者后置定语就省掉be。另外一种就是定语从句中引导词后面加be动词。希望对你有用。

Answer 2

一般情况都要加的,
A is/are (not) done (by B).
A is/are (not) going to be done (by B).
A will (not) be done (by B).
A has/have/had not been done by B.

被动语态中,使用有些动词时不用be,如read ,write,sell,wash等（单词本身就有被动的意义）,例：The shirt washes well.

This style of shoes sells well.

Answer 3

过去分词做后置定语时不要be 动词

其它情况都要
如
The bridge was built last year.

The girl called Amy is my friend. (called 做后置定语)