Display value only IF...

Hey, everybody.

Please look:

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I the values from column E are a result of the subtractions of column B and column D. And, as it happens, the column E is blank, because the time for its filling hasn't come yet, but yet I want to set the formulas now to set it all up ready, cause I have to hand the file on to be filled by someone else.

What I want to do is that the result column (E) will be blank UNTIL anything is put in column D, because, as you see it's a "Final Loss" field, and it does not make sense to have it displaying the SAME content in column B (which is only there because the formula I used was the simple =B7-D7, and, of course, as zero being now the content of column D, Column B minus 0 equals Column B itself). I just want Column E to be/display nothing until anything is put in Column D.

Just to put it in a easier way to understand:

Column B is Enrolled Students
Column D is How many students will have been back by February
Column E is How many students will not have been back by February

Since I can't know who will/won't be back, I need to have that field blank until I write who is back, and I can't just leave it the way it is because it doesn't make sense to have the same number for both Enrolled Students and Students who didn't come back.

Thank you very much. It was of great help.
And after it's done it looks so simple that I actually feel ashamed of having asked for help.

Do you have any advice for learning new commands, something nice and didactic, like a game, or a program-tutorial, challenges, or anything of that sort...?

Many thanks again.