How did you declare the global variable in the main form? If it's Public,
then your subform can see it. If it's Private, then the subform cannot see
it.

You cannot declare global variables in a form...their scope would be limited
to the module (in this case the form).  Global variables (if they should be
used at all) must be declared in a module.

There may be a way to do what you want without using a variable at all, but
your objective isn't very clear making it difficult to suggest things.  For
example, if you're just trying to get the value for a field in the parent
form and use it in your subform, you could use Me.Parent.SomeFieldName.  Or
you could create a property or function in the parent form that returns the
values that you want.  In any case, you should minimize the scope of any
variables you declare....to do otherwise is a bad programming practice.