Be动词和助动词Do,一句话什么时候用DO什么时候用BE?什么时候两个都不用?如:1.don't give up2.Are you.或what are you doing?.

be是一个连系动词,它有自己不同的形式
现在时：be 进行时：being 过去时：was/were 过去分词：been
它的后面必须跟形容词或名词作表语,与之构成系表结构,充当句子的谓语.
i am a student主语：i 谓语：am a student（系表结构）
being是它的现在进行时态,一般不单独用,而是用于现在进行时的被动语态.（being+动词过去分词）
the bike is being repaired.单车正在被修理中.
it is being finished by him这正在被他完成中.
been是它的过去分词,也不一般单独使用,而是用于现在完成时、过去完成时以及需要它的句子.
i have been here for a long time.我已经到这里有一段时间了.
至于你的问题,这与你想表达的意思有关.
你的工作是什么?
what do you do?what is your job?
那么这里选用do和is有什么根据呢?
第一句里面,第二个do是指你做什么,也就是你的工作,就是一个实义动词.按中文说,what you do已经足够了.可是英语里面就不行,必须跟一个助动词,也就是do.
这里就有了一个结论：当句子中有动词时,我们就用助动词do或是does.如果没有动词时,我们就选用is或是are,也就是b“Be”除了原形的“be”之外,还有另外七种形式：am,is,are,been,being,was,were.
在句子中,“be”可以是主动词(The Principal Verb)或助动词(The Auxiliary Verb).
当主动词时,“be”在性质上属于接系动词(The Linking Verb),后面要有名词、形容词、地方副词或短语作补足语(The Complement).例如：
1.The man is a science teacher.
2.Mary's new dresses are colourful.
3.I have been there before.
4.Mother is in the kitchen now.
这四个都是陈述句,可以变成疑问句,方法简单,把主语和“be”或助动词对换位置即可：
5.Is the man a science teacher?
6.Are Mary's new dresses colourful?
7.Have I been there before?
8.Is mother in the kitchen now?
当“be”要在祈使句中出现时,它必须借重助动词“do”或“don't”之助,如：
9.Don't be silly!
10.Do be obedient!
11.Don't be a fool!
“Be”有两种缩写法,如下：
12.He's not./He isn't.
13.You're not./You aren't...
但“am + not”的缩写法只有一个：
14.I'm not.
有人用“ain't”,但这并不是标准英语.
谈过了“be”作主动词的功能,现在看看“be”作助动词时,有些什么用法：
1.“Be + 现在分词”以组成进行式时态(Continuous Tenses),如：
15.Tony's maid is washing his new car.
16.The children are playing in the field.
17.Samuel was eating when I came in.
18.We have been living here since 1959.
2.“Be + 过去分词”以组成被动语态(The Passive Voice),如：
19.Her money in the drawer was stolen.
20.A number of good jobs are taken up byforeigners.
21.David studied in * but has been trained as a language lecturer in America.
22.Can all the wonderful birds be found in our Bird Park?
23.The disobedient boy was told to stand out- side the classroom.
24.Steps are being taken to reduce traffic con- gestion during peak hours.
25.Dishonest students will be immediately dealt with.
当陈述句的谓语是实义动词时,助动词do通常用于疑问句和否定句中.如：
I go to school every day.
我每天上学.
Do you go to school every day?
你每天都上学吗?
I don”t go to school every day.
我不是每天都上学.
另外,助动词do还可用于强调句中,用于强调谓语动词.如：
将助动词do及do 的相应形式放于动词前,而动词要用原形.又如：
I did tell him.
我的确告诉他了.
He does like playing football.
他的确喜欢踢球.
其余时候do作动词原形,表示做.