What if later in life they come to a metaphorical door, like an opportunity（机会） of some sort, and instead of opening the door and taking the opportunity, they just stare at it and wonder, What if it isn’t a door? That is, What if it isn’t a real opportunity?
这个句子比较长，个人是如此解构的，不知是不是正确，请各位老师指正：
What if+状语later in life+主语they+谓语come to+宾语a metaphorical door+插入语like an opportunity（机会） of some sort,+状语and instead of opening the door and taking the opportunity,+主语they+谓语just stare at+宾语it+谓语wonder+宾语从句What if it isn’t a door?
主语That+系动词is+宾语从句What if it isn’t a real opportunity?
剖析结构要从大处着眼，先找出基本框架，然后逐个细分。
What if.....? ——这是一个主句省略的说法。主句不完整。补充完整是：
What will happen if....?
What can they do if...?
What （主句：“他们该如何解决”）if later in life they come to a metaphorical door, like an opportunity of some sort, and instead of opening the door and taking the opportunity, they just stare at it and wonder, What if it isn’t a door?（条件从句：“假如....”）
That is, What if it isn’t a real opportunity? （另外一句）
if 从句结构剖析：
if later in life （时间状语）they come to a metaphorical door, like an opportunity of some sort, （第一个并列分句）and instead of opening the door and taking the opportunity, they just stare at it and wonder, What if it isn’t a door?（第二个并列分句）
第一个并列分句：they come to a metaphorical door, like an opportunity of some sort, （door 的非限制性定语）
第二个并列分句：and instead of opening the door and taking the opportunity, （方法状语）they just stare atit and wonder, What if it isn’t a door? （wonder 的直接宾语）
