A scientific calculator you type into rather than click. Trigonometry, logs, factorials, combinations, degrees or radians, and a session that carries answers forward.
Why is -2^2 minus four and not four?
Because a power binds tighter than a minus sign, so it reads as -(2^2). That is the algebra convention and what every textbook means. Spreadsheets famously do the opposite and give 4. If you want the other answer, write (-2)^2 and the brackets settle it.
What does ans do?
It refers to the result of the previous line that worked. That turns a single box into a session: work something out, then build on it, then build on that. If a line fails, ans keeps pointing at the last good value rather than becoming an error itself.
How do I switch between degrees and radians?
With the angle mode control. It changes what sin, cos and tan take and what asin, acos and atan give back. Gradians are there too, where a right angle is 100 rather than 90. Getting this wrong is the single most common source of a wrong trigonometry answer.
Can I write 2pi without a multiplication sign?
Yes. Anything sitting next to a value multiplies, so 2pi, 3(4+1) and 2sin(30) all work. It binds exactly as hard as a typed multiplication sign, which means 1/2pi is half of pi. Some calculators make it bind tighter and give you 1/(2 x pi) instead, so use brackets when it matters.
What happens when a line is wrong?
It gets a message instead of a number, and the rest of the lines carry on. A square root of a negative number says so in words. So does a division by zero, a logarithm of zero, an arcsine outside -1 to 1, and a factorial of a fraction. NaN never reaches the screen, because NaN tells you nothing about what to fix.
How large a number can it hold?
Up to about 1.8 followed by 308 zeros, which is the limit of double precision arithmetic. Factorials stop at 170! for the same reason and say so rather than showing infinity. Results beyond fifteen digits are shown in exponential form because the digits after that are not meaningful.