1、如果是一般过去时，就用was/were
2、如果是一般现在时，就用am/is/are
3、如果是一般将来时，就用will be
be动词，意思和用法很多，一般的意思是：是.此种用法，有多种变化形式，is，am，are，was，were，being，been，to be.另外，be动词还有成为的意思。根据句子中不同的人称、数和时间，时态，应该选择相应的be动词
1、be的时态变化 系动词be会有一般现在时、一般过去时、一般将来时、现在完成时、过去完成时等的时态变化。如：
He is a student.
They were in the park yesterday.
It will be cloudy tomorrow.
He has been ill for six days.
It had been three hours before she left the hat shop.
2、be与情态动词的连用 其用法为：情态动词+系动词be原形。如：
It must be Lucy's book. Her name is on the cover.
It can be very difficult for us to memorize too many words within such a short time.
She should be here on time tomorrow.
（二）助动词be
作为助动词，be没有具体含义，而是与主要动词一起构成句子的谓语动词。这种情况下，be主要用来构成进行时和被动语态。
1、进行时态中的be 此时，be可以与主要动词的现在分词一起，构成各种进行时态。如：
He is reading in the library now.（现在进行时）
She wasswimming in the river this time yesterday.（过去进行时）
They will be arriving in Beijing tomorrow.（将来进行时）
We have been learning English for ten years.（现在完成进行时）
2、被动语态中的be 助动词be能够与主要动词的过去分词一起构成被动语态，即be done结构。如：
Chinese iswidely used around the world nowadays.
The window wasbroken by the naughty boy just now.
Your watch has already beenrepaired.
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