to understand the problem is very difficult.(用it作为形式主语）—— ——

to understand the problem is very difficult.(用it作为形式主语）—— ——very difficult____ underst

to understand the problem is very difficult.(用it作为形式主语）—— ——very difficult____ underst
It is very difficult to understand the problem
形式主语：It
系动词：is
表语：very difficult
动词不定式做正真的主语：to understand the problem