I know both are adverbs and mean ‘indeed’, confirming the veracity of some condition or situation. I can’t find any difference in the way they are commonly used.
Does anyone know what the difference is? Thank you so much!
Both 实在 and 确实 can mean indeed or really. However, 确实 sounds more factual, so it's often used in a formal context. E.g. 经证实,此事确实与他无关。实在 sounds a bit 'emotional' or 'subjective' (增加了一点个人的感情色彩) and it's often used in informal or casual speech. E.g. 我实在是不知道!(it could imply that I really don't know and please don't ask me again, please trust me, and etc.) If you say 我确实是不知道,you try to emphasize the fact that you don't know (may not necessarily have any other implications).
On the other hand, 实在 has other meanings. E.g. 他这个人很实在 means he is quite honest. You can check on more meanings in a dictionary.
Both 实在 and 确实 can be translated as "really"
确 in 确实 refers to 确定 (confirmed) therefore 确实 can also means "indeed" (confirm a claim or suspicion)
Example:
"经调查后得知你确实不在场" (After investigation, we learned that you indeed were not present at the scene)
If you replace 确实 with 实在, you lose the "confirm a claim or suspicion" sense in the sentence
Though widely switchable in a lot cases, there are subtle differences. Just take a simple sentence as example:
这实在太难了。
这确实太难了。
In most cases, 实在 tend to express more subjective feeling while 确实 tend to indicate more objective judgement.
Also just to mention that 实在 can also be used as an adjective, though there's no much relation with our comparison here.
(...)确实(...) indeed (with evidence/reasons) or true
whether or not you believe it, doesn't change the fact that something is indeed...
...实在... truly believe (from personal experience/reasons)