Thanks for the help , masaegu san and MMM san.
I wrote "だ" because I thought that "だ" is equivalent to "です". Thanks for telling me that it was incorrectly to add "だ".
By the way, I didn't know that ”あの人は
おかしい" means "The person is crazy". Always thought that it means either "That person is funny" or "That person is strange/weird", so it was a surprise to know that it means "Crazy" as well.
I watched a movie lately and there was this guy in the movie who hates his wife's brother and told her wife that "Jimmyはおかしい" then I thought he either said "Jimmy is strange" or "Jimmy is funny". Now I am pretty convinced that he is actually calling Jimmy crazy.
