It is best not to do some thing. Here, we possess the adjective best, but this adjective is hooked up to no noun. The adjective best is Utilized in a copular construction Using the dummy pronoun it. This pronoun does not refer to any object. There is not any noun https://share.evernote.com/note/74e6f5bf-4e0f-8a5f-1a27-70e011ebb01c