Bestimmt vs Selbstverständlich vs Natürlich

https://forum.duolingo.com/comment/28393962/Bestimmt-vs-Selbstverst%C3%A4ndlich-vs-Nat%C3%BCrlich

"bestimmt" is often used to reassure someone when there's in truth some doubt about what's going to happen / about the way things are:
"Mach dir keine Sorgen, bestimmt hat er sich nur verlaufen / er kommt bestimmt später." "Don't worry, I'm sure he only got lost / I'm sure he'll come later."
...or it can mean "definitely": "Auch wenn es später wird, ich komme bestimmt." "Even if it gets a bit late, I'll definitely come." - but personally I would only use it this way when talking about myself, otherwise it could be misunderstood to mean "I'm sure he'll come [= I believe he's likely to come]". I'd probably say "Er kommt ganz sicher" instead for "He'll definitely come".

 

"selbstverständlich" is stronger (and a bit more formal, sometimes more servile) than "natürlich" and is often used to strongly oppose doubt; e.g.: to imply that things were different would be an offence to your honour.
"Selbstverständlich bieten wir Ihnen auch individuelle Hochzeitstorten an." "Of course we also offer you individual wedding cakes."
"Darf ich den Käse probieren?" "Selbstverständlich!" "May I sample the cheese?" "Of course!"
"Sie bekommen das Paket selbstverständlich trotzdem noch vor Weihnachten." "I assure you that despite this, you'll still get the package before Christmas."
(cf.: "Das versteht sich von selbst!" = "That goes without saying!")

 

"natürlich" is the most common and neutral word.
"Natürlich habe ich wieder meine Schlüssel vergessen." "Of course I forgot my keys again."
"Wir haben natürlich höflich abgelehnt." "We politely declined, of course."
"Ich weiß darüber natürlich nicht so viel wie du, aber ich denke, ..." "Of course I don't know as much about it as you do, but I think..."