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be动词的一般现在时有三种形式，即：am, is, are。 1. 如果主语是第一人称I（我）时，be动词用am。 如：I am a student. 我是一名学生。 I am 还可缩写成I'm。如： I'm David. 我是大卫。
2. 如果主语是you （你，你们）, they （他们，它们，她们）或名词复数 （两个以上的人或物）时，be动词必须用are。 如：Are you twelve? 你是十二岁吗？
Tom and Lily are good friends. 汤姆和莉莉是好朋友。 They are at school. 他们在学校。
are与主语还可缩写。如： We are= We're， They are =They're, You are = You're。而are与 not可缩写成aren't。
如：They aren't students. 他们不是学生。但是am与not不能缩写。
3. 如果主语是单数名词、不可数名词或单数第三人称代词（he, she, it）时，be动词用is。 如:My mother is a teacher. 我的妈妈是一名老师。 He is a student. 他是一名学生。
is也可与主语缩写,如: He is = He's, My mother is = My mother's等。但是This is不可缩写。而is与not可缩写成 isn't。
如:This isn't a book. 这不是一本书。
根据以上叙述我们可以把动词的用法以口诀的形式表述出来： 我（I）是am, 你（you）是are,
剩下is留给她（she）, 他（he）, 它（it）, 两个以上都用are。
英语的be 动词是个用法比较复杂的动词。复杂的原因有两点：
1、 除了原形的 be 之外，对于不同人称代词以及单数名词和复数名词，be 会有各种变化形式和缩写形式。概括一下有七种形式：be, am, is, are, was, were, been, being。它们与人称代词和名词单、复数的搭配关系是：
现在时 I am, you are, he/she/it/ is, we/you/they are；名词单数、不可数名词用is，复数用are。 缩略式 I'm, you're, he's, we're, you're, they're 否定缩略式 I'm not, isn't, aren't
过去时 I was, you were, he/she/it/ was, we/you/they were；名词单数、不可数名词用was，复数用were。 否定缩略式 wasn't, weren't
过去分词 been 现在分词 being
2、在句子中，be有两种主要作用：一是作为系动词(The Linking Verb)，本身可以作用谓语动词；二是做为助动词(The Auxiliary Verb)，与谓语动词一起构成各种时态和被动语态等。
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下面做个简要的讲解。
一、be 动词做系动词
1、系动词+表语”的结构
当be 动词做系动词使用时，主要构成“系动词+表语”的结构，在句子中做谓语。表语可以是名词、形容词、副词、介词短语等。例如：
The man is a science teacher. Mary's new dresses are colourful. Mother is in the kitchen now.
I have been there before.
They shall be here at 10:00 tommorow. That can't be true.
You are not being very polite.
Your brother is being very annoying this evening
2、be 动词的一般疑问句，方法是把be 移到主语前面，也可说成是移到句首。
Is the man a science teacher? Yes, he is. / No, he isn't.
Are Mary's new dresses colourful? Yes, they are. / No, they aren't. Was mother in the kitchen then? Yes, she was. / No, she wasn't. Were you at home the day before yesterday? Yes, I was. / No, I wasn't. Was she late this morning? Yes, she was. / No, she wasn't.
3、be 动词用在特殊疑问句
在特殊疑问句里，be 动词仍然移到主语前面，但特殊疑问句是以特殊疑问词开头的，所以be 动词只能说是移到主语前面，或者说是在特殊疑问词的后面。例如：
Whose bike is broken? Xiao Ming's bike is. Who was singing in the room? Mr. Zhang was. Where are you from? I am from Wuhu. What class were you in? We were in Class 2. How old is Tom? He is ten.
4、be 动词的否定句
be 动词做谓语时，它的否定形式是在be 后加not，并且可以缩写。例如：
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It is not sunny today.
Tom and his friends are not in the park.
You were not nine years old when I went to the university. He was not often late for his class when he was a student. I wasn't here yesterday.
My parents weren't at home last Sunday.
5、be 动词的祈使句
be 动词的祈使句有肯定和否定两种形式。肯定形式是以be 动词开头，而否定形式或强调句形，要加don't 或do。例如：
Be careful! Be a good boy!
Don't be silly! Don't be a fool!
Do be obedient! Do be careful.
二、be 动词做助动词
助动词be 有两个基本用法，一是与谓语动词一起构成各种时态，二是构成被动语态。
1、“be + 现在分词组”成各种进行式时态。例如：
Tony's maid is washing his new car. The children are playing in the field. Samuel was eating when I came in. We have been living here since 1959.
This time next day they will be sitting in the cinema.
2、“be + 过去分词”组成被动语态。例如：
Her money in the drawer was stolen.
A number of good jobs are taken up byforeigners.
David studied in Taiwan but has been trained as a language lecturer in America. The disobedient boy was told to stand out- side the classroom. Steps are being taken to reduce traffic con- gestion during peak hours. Dishonest students will be immediately dealt with.
3、being 的分词短语
be动词的一般现在时有三种形式
