二．情态动词后跟完成式和进行式的用法

　　1.情态动词后跟完成式，表"原本应该干某事，而实际上没干"

　　Ishouldhavefinishedtheworkearlier.

　　Heisn'there.Hemusthavemissedthetrain.

　　2.情态动词后跟进行式，表示"想必正在……"，"可能正在……"，"应当正在"等意。

　　It'stwelveo'clock.Theymustbehavinglunch.

　　Theymaybediscussingthisproblem.

　　Hecan'tbetellingthetruth.

　　Sheshouldn'tbeworkinglikethat.She'sstillsoweak.

　　三．情态动词的同义转换.

　　1.can=beableto

　　2.must=haveto

　　3.needn't=don'thaveto

　　4.needdosth=needtodosth.