應(yīng)該這樣說：表示猜測(cè)時(shí),can以及can的過去式could都可以對(duì)過去或者現(xiàn)在的動(dòng)作、狀態(tài)進(jìn)行否定的猜測(cè).但后者語氣弱一些.
對(duì)現(xiàn)在進(jìn)行猜測(cè)：can/could not be doing sth.
對(duì)過去進(jìn)行測(cè)測(cè)：can/could not have done sth.
根據(jù)猜測(cè)的把握性大小進(jìn)行排列：
must have done sth.
can't have done sth.
could have done sth.
may have done sth
might have done sth.
這種用法中,情態(tài)動(dòng)詞本身的時(shí)態(tài)形式和動(dòng)作發(fā)生的時(shí)間沒有關(guān)系.