I got an HP-35 a couple of days ago, and it doesn't make sense. As you might know, there is no equal key only an "enter" key. I did [5] [x] [5] [Enter]. It just showed "0" as a result. I experimented with it and nothing seemed to work. I decided to watch a YouTube video, and what the guy did was something like this: [9] [Enter] [10] [Enter] [+]. It seemed to work for him. I tried the same thing as him doing 8 x 9. For some reason the result was 81. I tried 7 x 6. It displayed 56. What am I doing wrong? I also looked at the back label, but there was no info about actually entering numbers. Mine didn't come with a manual so I can't look at that. I'm so confused! Any help would be appreciated!

It sounds like you expected the calculator to use algebraic notation (which people customarily use), but the HP-35 (and mechanical adding machines, it seems) uses postfix notation. Archive.org has the manual for this calculator so you should probably review that.

Thanks so much for your help! I knew that it used RPN, and I thought I knew how to enter it. It turns out what I was doing wrong had a simple fix. I did [8] [Enter] [9] [Enter] [x], but the correct way was
[8] [Enter] [9] [x], without the enter after the second number. Thanks again!