adverb
1
BAD : All of a sudden the engine started to make a strange noise. Certainly, I stopped the car at once to see what had happened.
GOOD : All of a sudden the engine started to make a strange noise. Naturally, I stopped the car at once to see what had happened.
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Certainly is mainly used to emphasize that something is really true, really happened, etc: 'I'm sorry if I upset you. I certainly didn't mean to.' 'The file certainly wasn't given to me or it would be on my desk.'
When you mean 'as anyone would expect', use naturally or of course : 'It was the first time the little boy had seen an elephant and naturally he was a little scared.'
2
BAD : Are you sure that you certainly don't want to go?
GOOD : Are you sure that you definitely don't want to go?
BAD : If they certainly can't find a job, they should be given further training.
GOOD : If they definitely can't find a job, they should be given further training.
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When you mean 'absolutely certain and without even the slightest doubt', use definitely . This word gives very strong emphasis and is often used in connection with intentions and future events: 'He definitely wants to be a vet.' 'Do you think that you'll definitely be able to come?'
3
BAD : Certainly I think so.
GOOD : I certainly think so.
BAD : The car can be repaired certainly.
GOOD : The car can certainly be repaired.
BAD : Certainly, it was a pleasant surprise.
GOOD : It was certainly a pleasant surprise.
GOOD : It certainly was a pleasant surprise.
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Certainly is usually used like a middle position adverb: 'She certainly likes you.' 'His work has certainly improved this year.'
For extra emphasis, however, certainly may be placed before the first auxiliary verb and before be when this is the main verb: 'His work certainly has improved this year.' 'She certainly is one of the best teachers on the staff.'