4Biber,D.S.Johansson,G.Leech,S.Conrad & E.Finegan.Longnan Grammar of Spoken and Written English[M].Beijing:Foreign Language Teaching and Research Press,2000.
5Declerck,R.Grammar of the English Verb Phrase:Vol ume 1:The Grammar of the English Tense System[M].Berlin:Mouton de Gruyter,2006.
6Ehrman,M.The Meaning of Modals in Present-dayAmerican English[M].The Hague:Mouton,1966.
7Longman Dictionary of Contemporary English[M].Lon don:Longman,1978.