예) 돈이 많이 있나 봐. I guess/presume that he has a lot of money. 내 생각엔 그는 돈이 많은것 같아. 돈이 많이 있었나 봐. I guess/presume he HAD a lot of money. 내 생각엔 그는 돈이 많았던 것 같아. 돈이 많이 있던나 봐. I guess he used to have a lot of money.. (truthfully I haven't even studied 던 yet, so if this is wrong please don't worry about it) 내 생각엔 그는 돈을 많이 가지고 있었던 것 같아. 돈이 많이 있나 봤다. I guessed/presumed [before] that he has a lot of money (or a totally different meaning???) 난 그가 돈을 많이 가지고 있을것이라 생각했었어. If you can read Korean and understand what I write, I am sure that you can understand the difference between them. If you have a questions regarding that, let me know.

Adding more to KISON RYU's great answer,^^/ like the book says, the meaning changes in the past tense. The past tense of "-ㄴ가/나 봤다" would be "-ㄴ가/나 봤다(=-ㄴ가/나 확인해 봤다)" meaning "I checked (to see) if~" For example, I checked to see if she was still asleep. = 그녀가 자고 있는지 봤다. = 그녀가 자고 있나 봤다. = 그녀가 자고 있는지 확인해 봤다. = 그녀가 자고 있나 확인해 봤다. Checking to see if your sentences are correct,^^/ (besarlalluvia님의 문장이 맞는지 보면...) 돈이 많이 있나 봐. I guess/presume that he has a lot of money. ☞ Correct! 돈이 많이 있었나 봐. I guess/presume he HAD a lot of money. ☞ Correct! 돈이 많이 있던나 봐. I guess he used to have a lot of money.. (truthfully I haven't even studied 던 yet, so if this is wrong please don't worry about it) ☞ should be "돈이 많이 있었나 봐" = "돈이 많이 있었던것 같아" 돈이 많이 있나 봤다. I guessed/presumed [before] that he has a lot of money (or a totally different meaning???) ☞ Yes, it's totally different. It would be "I checked to see if he had a lot of money". \^o^/

The expression " 돈이 많이 있던나 봐" it is short version of " 돈이 많이 있었던 나봐" which emphasize a little more of the fact that he may had a lot of money in the past than previous one as " 돈이 많이 있었나봐" Do you agree?