In general, it is best to bind your form to one table.
Use a subform for entering into a related table.
Use combo boxes for seeing the values in lookup tables.
There are many factors at play when you combine tables in a query, such as
the number of tables, the type and direction of the joins, whether the joins
are on fields that are uniquely indexes, the clauses in the query (e.g.
GROUP BY), and the presence of other fields (such as subqueries.)
If you wish to read more about that, see:
Why is my query read-only?
at:
http://allenbrowne.com/ser-61.html
Additionally there are factors relating to the form itself, e.g. Recordset
Type, Allow Edits, user permissions, Windows permissions, drive media, and
so on.
