Referencing a control on an Access form is easy. Referencing a subform and its controls is a little more complex. To Access, a subform isn't a form at all, but a control, so you must reference it as such.
One of the most common mistakes I see, and hear about it, is improper syntax when referencing controls on a subform. As far as Access is concerned, a subform is just another control on the main form -- and that's what confuses folks.
First, let's review referencing controls in general. Use either of the following syntax statements to reference a control on a main form:
(In more recent versions, you can substitute bang (!) with dot (.) between objects.)
To refer to a subform or a control on a subform, you must remember that Access treats the subform as a control. Essentially, you have a form with a control with a control. To express that arrangement in terms Access can decipher, you need the Form property as follows
In other words, subform is simply a control on the main form.
What I commonly see is a simple transposition of the Form property and subform, which generates a runtime error:
In this form, Access assumes Form is a control. When it can't find a control named form, Access returns an error.