sicher und sicherlich!
what is the difference between sicher and sicherlich?
-Du bist sicher. -Die Arbeit wird sicherlich schwer. -Es wird sicherlich anstrengend. -Es wird sicherlich ein heißer Tag. -Hast du mein Geld morgen sicher? -Sicher ist sicher. -Du bist sicherlich schon 18, oder?
I'm german, and I'd use it like that. Yes, probably it wouldn't make much of a difference if you'd change "sicherlich" with "sicher". But "sicherlich" sounds more like something that isn't 100%. Like tomorrow it's "probably" hot or he can "probably" help you". So it doesn't really matter, don't think about it, think about more important things. :)
"Sicherlich" can never be adjective. "Sicher" can be both, just as how "sure" can fulfill both functions in English as well.
Sicher and sicherlich can be used interchangeably if to mean sure, surely, for sure, etc. But you must use sicher if you want to express secure, safe/safely.
There may be some exceptions, though I can't think of any right now.