确定性副词会透露讲话者对某件事的确信度。确定性副词会出现在主要动词前，但如果主要动词是 Be 动词时则放在后面。这类副词有 certainly丶definitely丶probably丶undoubtedly丶surely 等。
- He definitely left the house this morning.
- He surely won't forget.
- He is probably in the park.
- He is certainly a smart man.
- He has certainly forgotten the meeting.
- He will probably remember tomorrow.
- He is definitely running late.
- Undoubtedly, Winston Churchill was a great politician.
- Certainly, I will be there.
- Probably, he has forgotten the meeting.
当确定性副词 surely 放在句首，就表示讲话者认定某件事是真实的，但期望得到确认。
- Surely you've got a bicycle.
- Surely you're not going to wear that to the party.