当it作形式主语,后面用形容词作表语,即在it is + adj.这样的句型中,当描述主语的表语形容词是advisable, critical,crucial, desirable, essential, imperative, important, incredible, inevitable, necessary, recommend, strange, urgent等形容词时,主语从句的谓语动词多用虚拟语气,如果是现在时,谓语动词用should/would +动词原形(should/would经常省略);如果是过去时,则用should/would + have done的形式。
参考资料:http://www.hjenglish.com/page/6983/