adverb
1
BAD : Honestly, we didn't play very well in the final.
GOOD : To be honest, we didn't play very well in the final.
BAD : Honestly, I don't really like her husband.
GOOD : To tell you the truth, I don't really like her husband.
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Use honestly when you want someone to believe that what you are saying is really true: 'I honestly don't mind where we go, as long as we go somewhere.' 'I was going to give it back to you, honestly.'
To show someone that you are about to tell them your own true feelings about something (especially feelings that you usually keep secret), use to be honest (with you), in all honesty, to tell (you) the truth, (quite) frankly or to be frank : 'In all honesty, I'll be glad when the children are back at school.' 'To tell you the truth, I don't think that the marriage will last long.'
2
BAD : I honestly say that I haven't seen anyone work so hard.
GOOD : I can honestly say that I haven't seen anyone work so hard.
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I can honestly say (WITH can ): 'I can honestly say this is the worst film I've ever seen.'