[milami88]

Hi,

I am having troubling understanding the usage of particle "ga", e.g in the following example:

ドアが閉しまります！
Doa ga shimarimasu!

Can't I say "doa wa shimarimasu" meaning something like: as for the door, it is closing.

My confusion is: ga is used to introduce something new. But the door is there, it exists, and it is about to be closed. So why use ga?