If you are using the OpenForm method, you could use the openargs parameter to pass data to the opened form. You would reference it in the other form with Me.OpenArgs.
This works best with just one piece of data to be passed; if you need more than one piece of data, you could separate them with a dilimiter and then parse it out of Me.OpenArgs. Or if it is multiple fixed width pieces of data, you could use the mid function to parse it.
Another way of doing it is referencing the field from the called form like this:
variable = Forms![MasterFormName].Form![SubForm]![ColumnName]
MasterFormName is the name of your master form
SubForm is the name of the subform object in the master form
ColumnName is the name of the column in the subform object