How sure: adverbs
The certainty dial runs from a soft guess to full commitment, and 'of course' takes the point for granted.
tabun
probably; maybe
多分 たぶん
The default hedge: it commits softly to any claim and is often closed with deshou, kamo or to omou.
tabun daijoubu da yo.
It's probably fine.
多分大丈夫だよ。
tabun daijoubu desu yo.
It's probably fine.
多分大丈夫ですよ。
osoraku is the stiffer, more formal 'probably'; tabun is fine in desu/masu speech.
kitto
surely; I bet
きっと
A confident guess about something you can't check, such as someone else's feelings, an outcome or the future.
kitto daijoubu da yo.
I'm sure it'll be fine.
きっと大丈夫だよ。
kitto daijoubu desu yo.
I'm sure it'll be fine.
きっと大丈夫ですよ。
To commit to your own plan, say zettai iku (or kanarazu iku); kitto iku sounds like a heartfelt promise rather than a plan. kitto kite ne (be sure to come) is fine.
zettai
definitely; absolutely (not)
絶対 ぜったい
The strongest commitment either way, as in zettai iku (definitely going) and zettai dame (absolutely not). It is constant in promises and refusals.
zettai daijoubu da yo.
It'll definitely be fine.
絶対大丈夫だよ。
zettai daijoubu desu yo.
It'll definitely be fine.
絶対大丈夫ですよ。
zettai ni is the same word; casual speech usually drops the ni. For polite commitments, kanarazu (without fail) is safer, but it doesn't do 'never' the way zettai ikanai does.
mochiron
polite mochiron desuof course
もちろん もちろんです
Says something goes without saying, either before a statement or alone as a warm 'of course!' reply.
mochiron iku yo.
Of course I'm coming.
もちろん行くよ。
mochiron ikimasu yo.
Of course I'm coming.
もちろん行きますよ。
As a reply to a request, mochiron! means 'sure, happy to'.