C#-Closing Form and open new form

Dear All,

I am working with C# Language. i'm wondering, i have Login form for starting up and main form. i used code start Main form and close login form if user name & password is correct. Unluckily, my application is closed after login form is closed. I want to do this way, if my login form is closed so main form is still openning.

How to do that?

Please advice....

Regards,

Ratana