什么时候要用b动词 比如说with many friends absent和with many friends were absent哪个正确啊、、、

题目中前者是正确，因为with这个词，它后面跟上的应该是名词或者是一个名词性从句，而不应该是一个句子，前者many friends absent 是一个名词性从句，后者many friends were absent是一个含有主谓宾的句子，不能跟在with后面，所以后者是错误的。
而你这个be动词要什么时候用问的有点宽泛，be动词也是作为一个动词，可以做宾语，可以做to be 、should/could/would...be等等，具体问题具体分析。解释起来比较清楚。

with many friends absent 正确。其实这是with　复合结构。with　＋　宾语　＋　形容词或者过去分词；或者现在分词，或者介词短语等等。

因为with　是介词，所以不能出现句子。
with a book in his left hand
with his eyes looking at the sky
with his hands tied behind
楼主，这就是这种结构的用法
be 动词。be + adj i am a teacher.
be + doing/ done
he is cleaning the room.
the room is being cleaned by him
be 动词的用法，我不清楚你具体问的哪一种用法。请详细说明一下你的困惑。