What's wrong with my formula?

Question

In Google Sheets: First, I'd like to return the values in the whole column of D into Column H if:

1. E and F equal A and B
2. C is not blank

This was the formula I tried: =IF(ISBLANK(C2=TRUE),"",(ARRAYFORMULA(VLOOKUP(E2:E&F2:F,{A2:A&B2:B,C2:D},4,FALSE))))

Second, I'd like to return the values in the whole column of D in Column I if:

1. E and F equal A and B
2. C is blank

Here's what I tried: =IF(ISBLANK(C2=TRUE),ARRAYFORMULA(VLOOKUP(E2:E&F2:F,{A2:A&B2:B,C2:D},4,FALSE)),"")

Neither work though... What am I doing wrong? Here's an image of the spreadsheet:

Answer 1

Update

=MAP(E2:E,F2:F,LAMBDA(e,f,
    {SUM(IFNA(FILTER(D:D,A:A=e,B:B=f,C:C<>""))),
SUM(IFNA(FILTER(D:D,A:A=e,B:B=f,C:C="")))}

))

Answer 2

To get aggregate results, use vlookup() and query(), like this:

=arrayformula( 
  if( 
    len(E2:E & F2:F), 
    iferror( 
      vlookup( 
        E2:E & F2:F, 
        query( 
          { A2:A & B2:B, isblank(C2:C), D2:D }, 
          "select Col1, sum(Col3) where Col1 is not null group by Col1 pivot Col2", 
          0 
        ), 
        { 2, 3 }, false 
      ) 
    ), 
    iferror(1/0) 
  ) 
)

The formula goes into cell H2 and will fill the whole column. Clear columns H2:I before you apply the formula to make room for the results.

(outdated): To get row-by-row results, use Boolean arithmetic, like this:

=arrayformula( 
  if( 
    (E2:E = A2:A) * (F2:F = B2:B), 
    { 
      if( not(isblank(C2:C)), D2:D, iferror(1/0) ), 
      if( isblank(C2:C), D2:D, iferror(1/0) ) 
    }, 
    iferror(1/0) 
  ) 
)

See { array expressions } and arrayformula().