1. It is + adj. + to-infinitive: It is essential to exercise regularly.
2. It + linking verb (e.g. seems, appears, looks) + adj. + that-clause: It seems that he is not feeling well.
3. It + verb + obj. + to-infinitive: It takes a lot of effort to learn a new language.
4. It + verb + obj. + that-clause: It surprised me that she didn't know the answer.
形式主语的4种句型：
1. It is + adj. + to-infinitive：重要的是做运动。
2. It + 连系动词 (如seems, appears, looks) + adj. + that-从句：他看起来不太舒服。
3. It + 动词 + 宾语 + 不定式：学习一门新语言需要很大的努力。
4. It + 动词 + 宾语 + that-从句：她不知道答案让我感到惊讶。