Hi, Ashish Saraf
Thank you for posting in Microsoft Community.
Surface pro9 is not able to switch manually tablet mode / desktop mode,Under normal circumstances, if you separate the keyboard cover and the display screen, it will automatically enter tablet mode.,After you connect, you will automatically return to desktop mode,
It is impossible to actively enter and exit tablet mode using an external keyboard,There is a big difference between tablet mode and desktop mode,For example, in tablet mode, in order to facilitate you to click and operate, the icons in the taskbar and desktop will automatically adjust the size.In use, it's just a different way of operating.
Looking forward to your reply.
Huon | Microsoft Community Support Specialist