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初一英语题:关于系动词的问题

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在什么情况下句子中要加is,什么时候不用加is(要详细,造句不要太多)
解题思路: is是系动词be(am,is,are)中的一个每个句子都要有谓语动词有BE动词的句子叫做主系表结构,一般BE动词后面接形容词、名词或是分词,但不能接动词原形如果句中动词为实意动词,此时不用be(am,is,are)
解题过程:
is是系动词be(am,is,are)中的一个。每个句子都要有谓语动词。有BE动词的句子叫做主系表结构,一般BE动词后面接形容词、名词或是分词,但不能接动词原形。如果句中动词为实意动词,此时不用be(am,is,are)。
例如:The man is a science teacher.
He still loves you.

再给你介绍一下be的用法:
be 的用法口诀
我用am,你用are,is连着他,她,它;
单数名词用is,复数名词全用are。
变疑问,往前提,句末问号莫丢弃。
变否定,更容易,be后not莫忘记。
疑问否定任你变,句首大写莫迟疑。
be动词的用法:be (be/is/are/am/was/were)
vi
现在时 I am, you are, he is, we are, you are, they 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 was, we were, you were, they were (过去时否定缩略式 wasn't, weren't), 过去分词been, 现在分词being
英语的“be”是个特殊动词;有些语言,如马来文等,并没有“be”这样的动词。
“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)
例句对照
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.这个男子是一位科学教师
2. Mary's new dresses are colourful.玛丽的新衣服色彩鲜艳
3. I have been there before.我以前去过那里
4. My mother is watching TV in the room.母亲现在在客厅看电视 
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.
“be”作助动词时的用法:
【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.】“情态动词+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.