I want when I open the app,always ask the password first, and then switch to another screen. but when I enter the PW and restart the app, its 's in 2nd screen and it doesn't ask for PW. what should I do ?
Welcome to community. Please post a screenshot of your blocks. Maybe you use tinyDB to save credentials once a user sign in/login so next time your app doesn't ask for password again
Somewhat this way, I have not taken sign-up into considration, used fixed login-pass. You can adapt accordingly.
thank you.where is call logged in screen block? I'm new on mit app inventor idk where it is 
You have to start Companion or Build from Login Screen, not from others
This is just for illustrative purpose only.It is a place from where you will call a new screen or a new virtual screen - simply welcome screen logic.