確定性副詞會透露講話者對某件事的確信度。確定性副詞會出現在主要動詞前，但如果主要動詞是 Be 動詞時則放在後面。這類副詞有 certainly、definitely、probably、undoubtedly、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.
當確定性副詞 surely 放在句首，就表示講話者認定某件事是真實的，但期望得到確認。
- Surely you've got a bicycle.
- Surely you're not going to wear that to the party.