Answer 1

For example,
First of all give a name to the input box -
<input type="text" name="txtbox">

(<button onclick="press8">8</button>)

in javascript, you need to write like this -

function press8(){
txtbox.value += "8";
}

Answer 2

Means there is no functions named press7, press8 etc.
You need to make those functions using javascript.

Answer 3

You called the function but there is no functions