Following from my last post we should consider how you use script to point from an object in a main form to a specific row in a subform.
Here, life is not so rosy. Yes, this script will copy main form value to subform:
MySubform.MySubformField.Value := MainFormField.value .
MySubform.MySubformField.show () .
The trouble is, this will only update the first visible subform row. Now, if you're only displaying one subform record, that might not be an issue. But otherwise, whatever is the top row currently visible is the one that is targeted. Not the first in the dataset, note: the first visible. Scroll the list so that the top one pops off and you'll now be affecting the value in the row that was previously below.
Now, I'm not entirely sure why one would want to do such a thing in the first place, but no doubt someone will want to do something like this. I therefore post for completeness.
If the goal was to change the value in all sub records, you could do this using a procedure instead, followed by use of the RefreshScreen action (all existing records will magically be updated). You could also include the field you want to copy to the subform records in a relationship, and use that relationship for the subform.