Being happy,once it is realized as a duty and established as a habit,opens door

题目

Being happy,once it is realized as a duty and established as a habit,opens door
Being happy,once it is realized as a duty and established as a habit,opens doors into unimaginable gardens thronged with grateful friends.
opens doors这个很怪呀?从语法上如何分析呢?

答案

opens doors 的主语是Being happy,这是一个短语作为主语,用的第三人称单数形式.