In using TVM, I was told to always enter Present Value as a negative. Now I see that PYMT must also be entered as a negative in certain situations. What are the rules on using negatives with a TI BA PlusII model?

I'm not familiar with that particular calculator, but I don't think I've heard of anyone having to enter things as a negative. I know answers can come out of mine negative and I ignore it. The concept is that money can go both directions, and theoretically one of those directions is negative. For instance, for a loan you would get the money, and the payments would come back off that amount, the opposite direction. But do you need to see one of them as negative to get the concept?

If you want to experiment, use something you know the answer to and try it both ways and see what happens. (In other words, is it screwing it up if you don't use the negative?) For instance, given rate = 10%, periods (n) = 2. Do first a lump sum present value of $1000, which is 826.45. (My calculator gives that answer as a negative.) Then do an annuity, with 2 payments of $500 each (also 10%) and that present value is 867.76. If I enter any of those things as a negative, my answer comes out positive, but the same number.

Most of the time it won't make any difference. But if you're doing something like a net present value, the initial outlay of cash will be the negative and the present value of future cash flows a positive, which you net against each other. I don't worry about stuff like that cause I know they net against each other and don't need to see one as negative. So I guess it depends whether it's important to have those proper, or whether it's something that you can logically figure out whether it's something that needs to net against something else.

If that makes any sense.