create a query that works if you manually put in the test value, i.e. it would give you the records you want to appear on your subform. Then change the test reference in the query to Forms!frmYourFormName.txtComboBoxName and it should still work if the form is open with a value in the combo box showing. Then when in form design view, on the property sheet, choose the code builder for the AfterUpdate event for the combo. Between the Private Sub and End Sub lines that you should now see, enter
Code:
Forms("MainFormName").Controls("subformControlName").Form.Recordsource = "qryYourQueryName"
If you manage all this and it doesn't error out, you should not have to force Access to requery the subform, but if the records are not changing and the query worked before, add
Code:
Forms("MainFormName").Controls("subformControlName").Form.Requery
You could make the query a filter query and apply it as the subform filter property but I thought the suggestion would be easier.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.