Why does the % key not give me a percentage?
It is the modulo (remainder) operator, the same as in most programming languages: a % b returns what is left after dividing a by b. So 17 % 5 is 2. Pocket calculators overload the same symbol to mean percent, which is why this trips people up. For percentages, multiply by the decimal (0.15 for 15 percent) or divide by 100.
What functions and constants are available?
The trig functions sin, cos, tan and their inverses asin, acos, atan; the logarithms log (base 10) and ln (natural); plus sqrt, abs, and exp. The constants pi and e are recognised by name. Anything else, including hyperbolic functions or a base-2 log, is not supported and will be reported as an unknown function.
Why does 2^3^2 give 512?
Because exponentiation is right-associative, which is standard mathematical notation: the expression is read as 2^(3^2), that is 2^9. Most people expect this once it is pointed out, but spreadsheet software often disagrees, so bracket the expression explicitly if you are porting a formula from Excel.
Can I type an expression instead of pressing keys?
Yes, the expression field is a normal text input and it accepts the same syntax the keypad produces, for example sin(30) + 2^3 or sqrt(2)*pi. It is parsed on every keystroke, so the result appears without pressing Evaluate; the Evaluate button replaces the expression with its result so you can carry on calculating from it.
What happens with an incomplete or invalid expression?
Nothing is evaluated and the result field stays empty, with an error message such as an unexpected token or an unknown function. Division by zero produces a non-finite value, which is caught and reported rather than shown as Infinity. Because the parser is hand-written and never calls eval, a malformed input cannot do anything worse than fail.