Thanks for any help on this. 

I have created two forms/tables: property and building. building is child of property and has a lookup field of 'property name' in property table. I added a subform building to property mainfrom. My goal is:

1. when user edit an existing property, all the buildings associated with the property through the lookup field will be shown in the subform of the property.

2. when user add new building to an existing property, 'property name' is automatically filled in the subform of buildings when a new row is added.

Thank you very much.