为什么有些句子有be动词,有些没有be动词

为什么有些句子有be动词,有些没有be动词
比如说:
Mrs Anne Sterling did not think of the risk she was taking when she run after them.
I found my wallet outside my front door
第一句she run after them为什么没有be,第二句也没有
shi is running这句为什么又有be呢

我先说两句,be动词的用法有很多,就这个问题而言,第一句中由于run作为动词,句子已经很完整了.
而后一句is running表示现在进行时,是固定的形式,动词为is.
其实网上有详细解释的.
be动词的用法
be
[bi:]
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”是个特别动词；有些语言,如中文和马来文,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).
【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 Taiwan 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.