What are the reasons for the 2nd form not being modal?

>I have a modeless form that calls another modeless form. But when the 2nd form is opened I want the user not to be able to use to the 1st form. Once the 2nd form is closed, it is ok for user to use 1st form. The reference to the 2nd form object is a property of the 1st form.
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>I am thinking of the following approach:
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>Make 1st form invisible (hide) as soon as it opens the 2nd form. But I can't think of how to make the 1st form visible on closing of the 2nd form.
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>If you have any suggestions on how to accomplish this task, I would appreciate it very much.
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>Thank you.