I Assume you are using a relational model and bound forms and the parent form is bound to the parent record set, and the child form bound to the child record set.
If so, there should be a Key relationship - say ParentURN being a unique key in the parent data set, and ParentURN being a foreign key in the child recordset.
If you name these keys the same, the child form will automatically generate the correct key relationships for you when you add it to the parent. If not, in the data tab of the subform containing the child form, set the 'link child fields' to the child form dataset's foreign key and the 'link Master Fields' to the parent form key. Now when you navigate to a record on the parent, the child form will display only records related to the parent record.
