Synonyms for Certainly:

adv.

absolutely (adverb)
perfectly, positively, indubitably, unconditionally, completely, categorically, absolutely, Consummately, utterly, supremely, surely, unequivocally, conclusively, unqualifiedly, definitely, unrestrictedly.
affirmatively (adverb)
Declaratively, Affirmatively, assertively.
all (adverb)
sure, for sure, sure as shooting, surely, for certain.
categorically (adverb)
typically, undeniably, denominationally, flatly.
certainly (adverb)
steadfastly, obviously, Doubtlessly, indisputably, incontestably, undoubtedly, clearly, irrefragably, unquestionably, trustingly, plainly, incontrovertibly, veritably, assuredly, irrefutably.
reliably (adverb)
conscientiously, reliably, loyally, trustworthily, faithfully, dependably.
truly (adverb)
authentically, rightly, honestly, truly, factually, axiomatically.
without doubt (adverb)
absolutely, surely, positively, unquestionably, of course, assuredly.

adv

absolutely (adverb)
positively, categorically, Consummately, unequivocally, indubitably, unconditionally, unqualifiedly, Doubtlessly, completely, conclusively, supremely, absolutely, utterly, unrestrictedly, perfectly, surely, definitely.
affirmatively (adverb)
definitely, surely, positively, categorically, assertively, Declaratively, Affirmatively.
categorically (adverb)
indubitably, unqualifiedly, definitely, absolutely, typically, unconditionally, denominationally, undeniably, flatly, unequivocally, categorically, surely.
certainly (adverb)
surely, incontrovertibly, undeniably, undoubtedly, trustingly, unequivocally, clearly, steadfastly, incontestably, positively, plainly, Doubtlessly, definitely, absolutely, obviously, irrefragably, unquestionably, assuredly, irrefutably, veritably, indisputably, indubitably.
for sure (adverb)
for certain.
indeed (adverb)
veritably.
of course (adverb)
by all means.
positively (adverb)
categorically.
really (adverb)
authentically.
reliably (adverb)
conscientiously, reliably, loyally, trustworthily, faithfully, dependably.
truly (adverb)
definitely, axiomatically, honestly, truly, absolutely, authentically, factually, veritably, rightly.
unmistakably (adverb)
obviously.
without doubt (adverb)
surely.

n.

actually, precisely, literally, simply, evidently, essentially. as you wish, if you want, of course, help yourself, I don't see why not, yes, if you (really) must, by all means. limited, doubtless, certain. certainly (noun)
sure, for sure, sure as shooting, surely, for certain.

Other synonyms:

evidently, essentially, actually. simply, literally. precisely. of course. surely
yes.
without doubt
doubtless.
Other relevant words:
for certain, assuredly, sure as shooting, of course, essentially, literally, precisely, sure, by all means, certain, for sure, actually, simply, yes, evidently, limited, doubtless.

Usage examples for certainly

  1. " Certainly it's all right. – The Fighting Chance by Robert W. Chambers
  2. They certainly did not mind me! – How and When to Be Your Own Doctor by Dr. Isabelle A. Moser with Steve Solomon
  3. If anything could be done in a better way, we certainly wanted to know it. – The Victory At Sea by William Sowden Sims Burton J. Hendrick