How do I write the following condition in one cell (AA)?
cell 'C' - year bought
cell 'J' - bought commission
cell 'Q' - year sold
cell 'U' - sold commission
Cell 'AA' 2005 commission

if c4=2005,q4=2005 sum(j4,u4) if c4<2005,q4=2005 sum(0,u4)
if c4=2005,q4not=2005 sum(j4,0)

What is a good book that explain each function and gives good examples, I
can buy

This should do it entered into AA4:

=IF(AND(C4<2005,Q4=2005),U4,IF(AND(C4=2005,Q4=2005 ),J4+U4,IF(AND(C4=2005,Q4<2005),J4,"None
of these")))

If the combinations of C4 and Q4 are not any that you state, then you
will get the text "None of these" - you might want to set this to zero
or some other value.

Hope this helps.

Pete