I need help with this casio caculator

The key is marked "x-superscript-negative-one" and located just below the "SHIFT" key.

Press SHIFT MODE 7 and it will prompt for the number of digits.

Make sure you use the (-) key in the left column above the RCL key to enter negative exponents, not the - key which subtracts.

Do get all results in decimal instead of fraction notation, press SHIFT [SETUP] 2 to select LineIO mode.

Press SHIFT MODE 2 to select LineIO mode.

Hello,
It may be due to the angle unit used. You can have angles in degrees, in radians, and in grads. You should set the default angle unit to waht you problem calls for.
To set degree as default SHIFT MODE 3
To set radians as default SHIFT MODE 4
To set grads as default SHIFT MODE 5

The values of angles calculated by the inverse trigonometric functions are in the default unit that is set . This may be the source of the wrong answers that you get.

Hope it helps.

the way i enter my equations is a bit different from what you're doing, i use the fraction button to do division problems, i just feel it makes it easier to read.
anyway, the fraction button is on the far left, 3rd button down (it looks like a white box on top of a line, and below the line is another empty box). press that, and you can type 1.0x10^-14 on the numerator.
it seems that you haven't discovered the EXPONENT button. that button is on the same row as the fraction button, it looks like an x with a white box next to it. press that once you've typed out "1.0x10", and then type -14. the negative button btw is below the fraction button, and looks like this: (-)
press the right arrow twice on the circle thingy at the top of all the keys, and that'll advance you to the denominator. type 2.0x10^-5, using that nifty exponent key. press the equal sign and it'll solve it for you, all in scientific notation (the answer is 5x10^-10). the problem with this calculator is, it won't expand that answer for you with all the decimals and everything, but if its chemistry that shouldn't be a problem.
also, you can certainly use the "x10^x" button on the very bottom of the keypad (its next to the decimal button). by using this key, you won't have to press the exponent button since you can just type in the exponent directly next to the 10.

as for that negative log of 5.6, just input the negative, the log button is right next to the exponent button, and your number and yeah....it's that easy :]

i hope you like this caculator, it has so many functions (have you seen the size of the manual??)
good luck on your chemistry test!

From CASIO FAQS:

Why do I get the wrong answer when doing a percent calculation?

Make sure that you are following the correct procedure.
Adding 10%: 200 [×] 10 [%] [+]
Subtract 10%: 200 [×] 10 [%] [-]

Example 20% of 1200:
input 1200 then * 20 then press [ 2nd ] button then [=] (this mean the %) and =
1200 [*] 20 [2nd] [=] [=] =240.
Rate me if it's helped.

I do not know how to use my calaulator (casio fx-991).Please help me.

The y^x button is right next to the x^2 button. It is labled with an x raised to a square box (which I will write as x^box). Thus, if you enter (for example)

2 x^box 5 = 32