OK. So you have a form, called 'frmOffences', say. It is bound to a table called 'tblOffences', say. The table has, among other fields, three fields called 'Date_of_Offence' and 'Description' and 'personID'.
You have a Main form called 'frmPerson', say. The PK is 'personID'. Within this form you have a subform, bound to the 'tblOffences' table, called 'Offences_subform', say. It is a datasheet view perhaps. It is linked to the Main form by 'personID'. This subform only shows certain fields from the 'tblOffences' table (to make it fit in the window).
Put this code behind the field 'Date_of_Offence' in the subform.
Private Sub Date_of_Offence_DblClick(Cancel As Integer)
Dim strWhereCond as String
strWhereCond = "Date_of_Offence = " & Me!Date_of_Offence & " AND personID = " & Me!personID
DoCmd.OpenForm "frmOffences", , , strWhereCond
Forms!frmOffences.Description.SetFocus
End Sub
If your PK is 'carNumber' then you will need to change 'personID' with 'carNumber'.
If that doesn't work then there is something that you are not telling me!
: )