Let me try that again:
Hi Terence,
Depends on where Fin1 is - your code does not specify.
If Fin1 is on main form then:
Forms!mainform!subform.Form!Fin1Hidden = Forms!mainform!Fin1.Column(0)
If Fin1 is on the subform then:
Forms!mainform!subform.Form!Fin1Hidden = Forms!mainform!subform.Form!Fin1.Column(0)
Also, ditch the square brackets - not needed in code.