Period TwoLearning about Language情态动词()1can与could它们的否定式为can not/cannot,could not,简略式为cant,couldnt。主要用法如下:(1)表示能力或可能性。(2)表示许可。(3)can与be able to表示能力时,can常指现在,较be able to更常用。如果只表示能力,两者可互换,仅表过去的能力用could,但表过去的能力或特定行为时,用was/were able to;若指将来的能力则只能用shall/will be able to。be able to还可用于完成时态,而can不能。When he was young,he could sing very well.(表能力)他年轻时,歌唱得很好。Man can not live without air.(表可能性)没有空气,人不可能生存。You can sit here.(表许可)你可以坐在这儿。After finishing his courses,he will be