What’s the difference between 实在 and 确实?
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.