error 3122 in ACCESS -

I have a query which uses an expression... dividing "1" by a number field in
a static table.

I changed the expression to divide by a different number field from the same
table. This caused an error. 

Error 3122: I tried to execute a query that doesn't include the specific
expression < * > as part of an aggregate function. 

The possible cause is listed as: I didn't enter an aggregate function in the
TRANSFORM statement.

OK - The only thing different from this query and the one that worked is
which field is included in the expression. The expression is { NewFieldName:
(1/[FieldName])*([FieldCreatedbyanEarlierExpressioninQuery]) }.

1st - What is a transform statement?
2nd- What is an aggregate function?
3rd - Would I need to change anything is SQL... and what?
4th- Why wouldn't the same expression work with a different field?
            

Jack Schlobohm