UK:*UK and possibly other pronunciationsUK and possibly other pronunciations/ˈs3ːrtənli/US:USA pronunciation: IPA and respellingUSA pronuncation: IPA/ˈsɝtənli/ ,USA pronunciation: respelling(sûrtn lē)

WordReference Collins English Usage © 2020
emphasizing and agreeing
Certainly is used to emphasize statements. You often use certainly when you are agreeing with something that has been said or confirming that something is true.
It certainly looks wonderful, doesn't it?
Ellie was certainly a student at the university but I'm not sure about her brother.
Be careful
Don't confuse certainly and surely. You use surely to express disagreement or surprise.
Surely you care about what happens to her.
Both British and American speakers use certainly to respond positively to a question or statement.
‘Do you see this as a good result?’ – ‘Oh, certainly.’
American speakers also use surely in this way.
‘Can I have a drink?’ – ‘Why, surely.’
position in sentence
Certainly is usually used to modify verbs.
If there is no auxiliary verb, you put certainly in front of the verb, unless the verb is be.
It certainly gave some of her visitors a fright.
If the verb is be, certainly can go either in front of it or after it. It usually goes after it.
That certainly isn't true.
If there is an auxiliary verb, you usually put certainly after the auxiliary verb.
He'd certainly proved his point.
If there is more than one auxiliary verb, you usually put certainly after the first one. Certainly can also go in front of the first auxiliary verb.
He will certainly be able to offer you advice.
The roadway certainly could be widened.
If you use an auxiliary verb without a main verb, you put certainly in front of the auxiliary verb.
‘I don’t know whether I've succeeded or not.' – ‘Oh, you certainly have.’
You can also put certainly at the beginning of a sentence.
Certainly it was not the act of a sane man.
‘almost certainly’
If you think that something is true, but you are not quite sure about it, you can use almost certainly.
She will almost certainly be left with some brain damage.
Be careful
Don't put ‘nearly’ in front of certainly.
Adverbs and adverbials (for a graded list of words used to indicate probability)
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