How to say "Of course" in Japanese
I've heard many ways of saying "of course" and "surely" but am not sure which sound most natural or who uses which. Mochiron, touzen, souda, souka, yappari ne. Thanks.

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Of course: mochiron, touzen, machigainai (もちろん、当然、間違いない）
souda (as in そうです) = I agree
souka (as in そうですか) = Is that true? Really? I didn't know that! (also doubtfully: Really...? Dunno if I agree...)
yappari ne (as in やっぱりそうですね) = That's totally true, isn't it! I agree! (more formal: yahari やはり)

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もちろん。を、おおくつかいます。

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People always use "touzen"and"a tarimae".；）

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Completely depends on the context :) 
Touzen is more like, "no doubt"(like its already decided) and mochiron is more, "yeah, that is of course true but this blabla. . ."

I think you will learn the styles the more Japanese you study ^_^