When using Microsoft Excel I regularly choose to use the same sell
three or four times in different formulas.  So, I enter the cell
reference with a dollar symbol (e.g. $E$21) so that when I copy the
formula to another cell, the reference to the cell I want constant is
maintained.

I'm almost certain there is a keyboard shortcut so that when I'm
writing a formula (e.g. =B20+E21) and I select the E21 cell with my
mouse, as opposed to typing the cell reference into the formula, the
reference will automatically add the dollar symbols (e.g. =B20+$E$21).
 I've tried Control and Alt, but they don't seem to do the trick.

Hi jaceventura-ga,

This is a simple yet very useful trick for the purpose you've
described in your question. Here are the simple steps:

1. Go to the cell you want to enter the formula into
2. Enter the plain cell reference (either by typing the reference or
clicking a cell)
3. Hit the function 4 key (F4)

This will turn the adjacent cell reference from something like A1 -->
$A$1. You can also get to different types of references by hitting F4
multiple times. Examples of this would be $A1 and A$1.

Hope that helps!

answerguru-ga

That is exactly what I needed.  Thank you.