(1) It be adj. (for sb.) to do sth.此处adj. 通常为描述事件的形容词:easy, difficult, hard,necessary, unnecessary, possible, impossible, likely, unlikely, right, wrong,important, unimportant, legal, illegal, well-mannered, ill-mannered, polite,impolite, clear, obvious, certain, suitable, proper, fit, useful, useless,dangerous…
例 It is illegal (for a teenager) to drive a car without a license.