To sum the column named "Amount" in the subform, display the subform in
Continuous View, add a Form Footer section (View menu when in form design).
Put a text box in the Form Footer section, and set these properties:
Control Source: =Sum([Amount])
Format: Currency
If you wish to display the total on the main form instead, open the
Northwind sample database, the Orders form, and see how it picks up the
total from the subform.
You would not store this total in the main form's table. Doing so violates
one of the most basic rules of data normalization (not to store dependent
data), and will cause you unnecessary maintenance problems (to guarantee the
total is always right, no matter how the data is updated.)