1.introduce a sufficient condition: if/when/whenever/every/all/any/people who/in order to/to
2.introduce a necessary condition: then/only/only if/must/require/no,none(在句子开头)
3.unless equation: unless/except/until/without(其后面部分变成必要条件，其它加NOT变充分条件)
4.特例：the only way to achieve success is to work hard. achieve success(充分条件)
only a professional consultant can solve the organization’s problems. a professional consultant(必要条件)
no citizen can be denied the right to vote. denied the right to vote(充分条件). no citizen(必要条件) | 
