"Bu kitap gerçekten zor." Translation: This book is really hard.
Why not ..this book is indeed hard ?
I can see what you mean clearly & Duo did not accept your answer for this reason.
Really (adverb) - in actual fact.
Indeed (adverb) - really or certainly. Used to emphasise something that is already known. In this example (zor) - hard.
"Bu kitap gerçekten zor." Translation: This book is indeed hard. - Correct.
I like your answer & Duo may not so a like from me ^
I'm not a native speaker, but I think you're correct. Gercekten gives the sense of "in truth/reality" as if the speaker was confirming what either they or others had suspected, i.e., the book truly IS difficult. "Bu kitap cok zor" is closer to "This book is really/very hard".
"gerçekten" translates as "really" because it also means "certainly /actually". This is slightly different than "çok" which means "very" or "extremely".
So although we can often use "very" and "really" interchangeably in English, I think this course separates them so that we can learn the difference between "gerçekten" and "çok". :-)
There are situations in English, as well, when we can't swap them... For example: "You really are my friend." cannot be "You very are my friend."
quite = oldukça ; gerçekten = really /actually /genuinely
I can't speak for everyone, but in my dialect of English, "quite" is a middle-ground between "a bit" and "very". While "actually /really" is more certain-- as in, there is no debate or no middle ground, it's really a difficult book.