How to implement nested CASE statements in a SQL Select of a stored procedure?

I have a SELECT statement from a temp table in a stored procedure that selects these two columns:

DECLARE @Mode INT

CASE 
    WHEN t.Descr = '-- Prior Balance --' 
        THEN '' 
        ELSE t.ChgAmount 
END AS ChgAmount, 
CASE 
    WHEN t.Descr = '-- Prior Balance --' 
        THEN '' 
        ELSE t.PayAmount 
END AS PayAmount,

I want to conditionally return those two columns differently depending on the value of @Mode, specifically if it is equal to 7.

I'm getting confused about the levels of nesting that I need and the formatting of doing this.

So far I have tried something like this:

CASE 
    WHEN @Mode = 7 
        THEN 
            CASE 
                WHEN t.TranDate > @Due 
                    THEN t.ChgAmount 
            END	
END AS CurrentCharges,
CASE 
    WHEN t.Descr = '-- Prior Balance --'  
        THEN '' 
        ELSE  t.ChgAmount 
END AS ChgAmount,
CASE 
    WHEN t.Descr = '-- Prior Balance --' 
        THEN '' 
        ELSE t.PayAmount 
END AS PayAmount,  

The above SELECT might work, but it would stil return the extra column. How should I nest the other, original, CASE statement for the ChgAmount?

First, let's clarify in abstract terms that in the following pseudo-code

If X then
    Case 1
    If Y then
        Case 2
    Else
        Case 3
    End If
Else
    Case 4
End If

• Case 1 is equivalent to "X is true"
• Case 2 is equivalent to "X is true and Y is true"
• Case 3 is equivalent to "X is true and Y is false"
• Case 4 is equivalent to "X is false"

Furthermore, let's clarify that case-when criterias are logically very similar to our pseudo-code presented above of if-then, hence, you can apply composite criteria instead of nesting case-when if you prefer that, but also, you can implement nested case-when criterias, it's a matter of style.

As a result, you will need to formulate the logic you want to apply, by asking yourself the following questions:

• do I need the same number of fields in my different cases? (if not, then you will probably need to write different queries in different cases)
• what cases do I have for my fields if @Mode is 7?
• what cases do I have for my fields if @Mode is not 7?
• how can I merge my criterias in the points above into coherent (composite) criterias that would not require nesting?

If you answer these questions as an edit to this question, then we will be able to more properly answer your questions than the general terms I'm using in this answer and we may provide code for you as well. However, if you think this through, then you might also be able to implement this in a not nested way. And, if you are able to understand this as far as to implement it into a not nested way, then you could transform that implementation into a nested implementation as well.