can作推测时,只能用于否定.表示“不可能”.它的反义词---“可能”则用may/might.比如,
He can't be a teacher.(他不可能是老师)
He may be a teacher.（他可能是老师）
She can't be 13.(她不可能才13岁)
She may be only 13.（她可能才13岁）
另外,还可以对过去的事情进行推测,表示“某某不可能已经.”比如,
He couldn't have told her the truth.
他不可能已经告诉她真相了.