This means the app needs to remember the $$x$$ value and the pending operator until you click the $"="$ sign. Use the following steps to implement a basic calculator app...

Steps:

Disable the textbox so that the number keys are the only way to enter operands.

You need $3$ private Form variables:

x$,$ which is the first operand

pendingOperation, the operator to be applied between x and the next number

isNewNumber, which is true when $=$ has put a result into the textbox, and false as soon as a digit is clicked.

When a digit or decimal point is clicked $($note: these can all use the same event handler$)$:

If isNewNumber is true, clear the textbox & equation label, set isNewNumber to false

Cast sender to a Button so you can append its Text to the textbox: i$.$e$.(($Button$)$sender$).$Text

Ignore a second decimal place if the textbox already has one.

The operator buttons move the textbox$$s value to x$,$ clears the textbox, and sets pendingOperation to the operator displayed on the button. Again, these can all use the same event handler.

The $=$ button$$s event handler has a switch $($or a compound if$-$elseif$),$ applying the pendingOperation between x $($stored$)$ and y $($in the textbox$).$

The result is placed in the textbox & appended to the label, x is set to zero, pendingOperation is cleared, and isNewNumber is set to true.

The $12/5=2\mathrm{.}4$ label above the textbox is obvious, right? The main question is not $$how$...$ it$$s $$when$...$ i$.$e$.$ when are operands and operators appended to it$?$ And when is it cleared?

C and CE $($which is short for $$clear entry$)$ both clear the textbox, while C also clears the label above the textbox, then sets x to zero, pendingOperation to $,$ and isNewNumber to true.