i am trying to start a form named login. i have changed my start up form in properties but different form start when i click debug. can you help me???

It could be happen for many reason.
Don't attempted to show logIn Form from other FormClass. Check and remove it if you do it from anywhere.

Show the logIn form as Modal. If you try to show it modelessly your project shows the first form you attached in your project with this lofIn form.

Maybe it is in different project.

go to your Project>properties>Application then see if it is pointing to your Form.

This has happened for me more than once. I usually do delete all the files in the debug folders and run it again, it works everytime. "Clean solution" on visual studio will do this for you i think.

open The folder of your project then Execute the .sln file. and then open the solution explorer
Project>applications>startup form. change it.
make sure the project you open is the project you are seeking for.
lesson learn that multiple solution in one project is not recommended.