If this is the typical setup where the main form contains records and the
subform has only the matching records, it is not just a matter of applying a
filter to the subform. What you need to do is to filter the main form so it
contains only those records that have a match in the subform.

If that's what you are trying to do, see:
    Filter a Form on a Field in a Subform
at:
   http://allenbrowne.com/ser-28.html

If the subform has nothing in its LinkMasterFields/LinkChildFields
properties, then it is just a matter of applying a filter. You could put an
unbound text box on your form, and use its After Update event procedure to
filter the subform:

Private Sub txtFindThis_AfterUpdate()
   With Me.[NameOfYourSubformControlHere].Form
       If .Dirty Then
           .Dirty = False
       End If
       If IsNull(Me.txtFindThis) Then
           .FilterOn = False    'Nothing entered: show all records.
       Else
           .Filter = "[Subject Keyword] Like ""*" & Me.txtFindThis & "*""
           .FilterOn = True
       End If
   End With
End Sub

I read an earlier post about using filters and I couldn't get it to
work.  

The main form also has fields that my users will want to search: eg
title, author, journal details etc

The subform is connected to a query of TitleID & SubjectID, and which
is related to a table of subject keywords.  As I catalogue a
book/journal I search the table and select appropriate subject
keywords.  So that one book/journal article etc may have 1 or more
subject keywords attached to it.  

The users of the database may want to search for titles based on a
keyword.  

Does that provide enough information to ascertain whether filters is
the way I need to go?

thank you

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