What is the difference between 確かに and 誠に? I found that:
確かに means: certainly, indeed, surely, sure, definitely;
誠に means: indeed, truly, really, surely.
Looks like the meanings are almost the same. When each of those words should be used?
確かに and 誠に have very similar meanings. The difference between them is that 確かに is often used in everyday conversation, but 誠に. 誠に usually used in very formal situation or some historical period drama, because it sounds very formal and somewhat old. So I recommend you to use 確かに.
Here are some examples:
- 確かにそう思う。 (I surely think so; good usage)
- 誠にそう思う。 (I surely think so; somewhat sounds like an old people)
誠に is often used when a company or an organization made a serious mistake or have a serious problem and want to make apologise. They sometimes use "誠に遺憾" which means "truly shameful", to accept the problem is really serious. You could replace 誠に with 確かに, like "確かに遺憾", but it isn't formal enough for the situation.
Like in English, you can find a dozen words to mean the same thing. Maybe half of those could be considered in common use. And half again are the few words that you hear the most often to express that meaning. The best way to learn which word should be used in which situation is to listen to native speakers and see what word they use, when they use it, and how.
During my time studying in Japan, I heard 確かに used so much that I developed a habit to use it myself quite often. But I can't remember hearing 誠に used at all, at least not in colloquial conversation settings which is where I focused most of my study.
To give you a comparison with English, I would use 確かに to mean "of course" or "certainly" in a situation like the following:
I show my gun collection to some Japanese friends who visit me in America. They like shooting them, so I suggest they might look into getting a gun of their own in Japan. They reply that people in Japan don't own guns. Now, I know that it's uncommon for people in Japan to have guns, but it's not unheard of. So I reply, 「確かに、普通じゃないけど、田舎の人は時々ある。」 "Certainly, it's not normal, but country folk sometimes have them."
In contrast, 誠に sounds more formal and antiquated. I might use it to mean "truly", "earnestly", or "with certainty" only if I'm trying to sound poetic, or maybe to be funny or ironic. I might tell someone I love them truly 「誠に愛している」or that I trust them completely「誠に信じている」. Again, this is not a phrase that is used in casual conversation by normal, modern speakers.