是正对现在正在发生的事件的否定推测.
比如现在正是上班工作时间,我说:
你爸爸现在肯定正在公司上班.
Your dad must be working in the company now.= I'm sure you dad is working in the company now.
如果是否定的推测就说:
Your dad can't be working in the company now (because he is taking three days off these days.) = I'm sure you dad isn't working in the company now.