system preferences
I signed out form my Apple ID on my MacBook Air and when I signed in, it keep saying that t=system preferences is busy and can t close. I can t turn it off, what should I do?
That is strange.
Shutdown again. wait until it is really really off.
Then you startup while holding the Shift key, this is a very slow startup:
at the end it is in the SafeMode, login, try any app, then close it, shutdown again,
then startup again: practically all caches are now cleared.
It should startup correctly.