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二者时态不同，is是一般现在时态，was是过去时态。二者用法不同，“现在时间”用is，“过去时间”用was。be动词的形式有am，is，are，been，being，was，were。
be动词
be（be/is/are/am/was/were)[bi:]vi
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
“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
当做主动词时，“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：
The man is a science teacher.
这个男子是一位科学教师。
Mary's new dresses are colourful.
玛丽的新衣服色彩鲜艳。
I have been there before.
我以前去过那里。
My mother is in the kitchen now.
我妈妈现在在厨房里。
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：
Is the man a science teacher?
Are Mary's new dresses colourful?
Have I been there before?
Is mother in the kitchen now?