Frequency: The word is one of the 1500 most common words in English.
1.
You use ~ to emphasize what you are saying when you are making a statement.
The bombs are almost ~ part of a much bigger conspiracy...
Today’s inflation figure is ~ too high...
Certainly, pets can help children develop friendship skills.
= undoubtedly
ADV: ADV with cl/group emphasis
2.
You use ~ when you are agreeing with what someone has said.
‘In any case you remained friends.’—‘Certainly.’...
‘You keep out of their way don’t you?’—‘I ~ do.’
ADV: ADV as reply
3.
You say ~ not when you want to say ‘no’ in a strong way.
‘Perhaps it would be better if I withdrew altogether.’—‘Certainly not!’
ADV as reply emphasis