系动词系动词，亦称连系动词（Link Verb）。作为系动词，它本身有词义，但不能单独用作谓语，后边必须跟表语，亦称补语（形容词），构成系表结构说明主语的状况、性质、特征等情况。英语除动词be 之外，还有一些动词也可以做系动词，如表示状态或情况的：keep, look, feel, appear, lie, remain, seem, stay, smell, sound, taste 等；以及表示变化的： get, grow, turn, fall, run, become, go, 等。1. to be 可以加在 seem, prove, remain, appear 等系动词和其后的名词或形容词之间，但也可以省略。E.g. The task proved (to be) impossible. E.g. They seem (to be) such friendly people, but they never replied to our invitation. 注意：在 seem, appear 等词后，可加其他非to be 不定式，但这时他们不是系动词，而是不及物动词。Courtesy, politeness, good manner? Call it what you will, the supply never seems to equal the demand. 非人称代词it 做句子的形式主语1. 代表不定式 E.g. It is absurd to be afraid under such circumstances. 2. 代表动名词E.g. It is terrible trying to keep up with the Jones. 3. 代表 that 引导的从句E.g. It is plain to everyone that she was offended. 4. 代表 wh- 引导的从句E.g. As long as he finishes his work, it doesn ’tmatter when he comes to the office. 做句子的形式宾语1. 代表不定式E.g. They found it difficult to get along with him. 2. 代表从句E.g. I think it best that you (should) stay with me. E.g. He soon made it clear why he had asked a conference. 用来改变句子结构，使句子某一成分得到强调1. 强调主语E.g. It was she who put forward the suggestion. 2. 强调宾语E.g. it is that painting that he has sold at a good profit. 3. 强调状语E.g. it was through me that he has got the present job. 强调用 so 表示强调用 so 强调听者同意说者所言，这种用法只将so 提到句首，其他成分位置不变。E.g. –you have spilled coffee on your dress. - So I have. 情态动词 + 完成式could+have+过去分词有时表示过去...
