为什么当be是个助动词时，也有现在分词和过去分词?

为什么当be是个助动词时，也有现在分词和过去分词?

助动词be的用法

1） be +现在分词，构成进行时态，例如：
They are having a meeting. 他们正在开会。
English is becoming more and more important. 英语现在越来越重要。

2) be + 过去分词，构成被动语态，例如：
The window was broken by Tom.. 窗户是汤姆打碎的。
English is taught throughout the world. 世界各地都教英语。

3） be + 动词不定式，可表示下列内容：
a. 表示最近、未来的计划或安排，例如：
He is to go to New York next week.. 他下周要去纽约。
We are to teach the freshpersons. 我们要教新生。
说明： 这种用法也可以说成是一种将来时态表达法。
b. 表示命令，例如：
You are to explain this. 对此你要做出解释。
He is to come to the office this afternoon. 要他今天下午来办公室。
c. 征求意见，例如：
How am I to answer him? 我该怎样答复他？
Who is to go there? 谁该去那儿呢？
d. 表示相约、商定，例如：
We are to meet at the school gate at seven tomorrow morning. 我们明天早晨7点在校门口集合。

当be是助动词时，它的现在分词being和过去分词been都是助动词，没有词汇意义。 1. 助动词be的现在分词being的作用：动名词的被动形式和现在分词的被动形式是"being +过去分词"。 ❶ 动名词的被动形式"being+过去分词"在句子中可以作主语、表语、宾语等成分。 being praised by teachers is good for every student who works hard.受到老师表扬对于用功的学生来说是很有好处的。(主语) his thought is of being rewarded after he does anything good in his school.他的想法是在学校里做任何好事后都得到回报。(表语) he always thinks of being admitted to the key university after working hard these years.他总是想在这些年的努力学习后录取到那所重点大学去。(宾语) ❷ 现在分词的被动式"being+过去分词"在句子中可以作：谓语、补足语、定语、状语等成分。 he is being beaten by his father now.他现在在挨他父亲的打。(谓语) she was kept waiting for you for ages.她被迫等你都等了好久啦。(主语补足语) being built over the river, the bridge will be completed by the end of this month.那座桥正在河上建造，到月底竣工。(状语) the baby being taking care of in this nursery is the woman's who works in the nursery as well.在那家育婴室里照看着的那个婴孩是那个也在那家育婴室工作的那个妇女的孩子。(定语) 2. 助动词be的过去分词been的作用： ❶ "完成时的助动词+been + 现在分词"构成完成进行时的谓语动词和非谓语动词现在分词完成式。 he has been working here for days.许多天以来他一直在这里干活。(谓语) having been working on the project for a week, he has finished the work at last.搞这个工程一周，他终于完成了这项任务。(现在分词的完成式作状语) ❷ "完成时的助动词+been +过去分词"构成完成时被动语态的谓语动词和非谓语动词现在分词完成被动式。 he had been kept copying the text at home since his father had gone to work yesterday.昨天自从他父亲去上班后就一直要他在家抄写那篇课文。(谓语) having been saved from the river, the drowning boy was in a trance state in hospital.从河里救起来之后，那个溺水男孩还在医院里昏迷不醒。 (现在分词的完成被动式作状语)

为什么不能有，do也是助动词，do也是有现在分词和过去分词的。