It's a pity..的疑问It's a pity he couldn't visit Mary.这是个定语从句吧?I could drive a car by the time I was sixteen.这是什么从句?

It's a pity..的疑问It's a pity he couldn't visit Mary.这是个定语从句吧?I could drive a car by the time I was sixteen.这是什么从句?
It's a pity..的疑问
It's a pity he couldn't visit Mary.
这是个定语从句吧?
I could drive a car by the time I was sixteen.
这是什么从句?

It's a pity..的疑问It's a pity he couldn't visit Mary.这是个定语从句吧?I could drive a car by the time I was sixteen.这是什么从句?
一、“it is + 形容词表语 + 从句”叫做主语从句,其中
1.结构说明
* it 是形式主语,
* 从句是真实主语.
2.该结构的作用
如果不采取形式主语的办法,这个句子应该是如下的结构：
he couldn't visit Mary is a pity.
很明显,主语部分是个较长的句子,而表语却只有一个形容词,整个句子显得头重脚轻.为了避免这种头重脚轻得凝重句式,于是就把做主语的从句放在句尾,主语空出来的位置由形式代词 it 来充当.
英语里,如果主语由较长的不定式短语、动名词短语或句子充当的时候,常常会出现这种“垂帘听政”的句式结构,而帝位则由“傀儡皇帝”it“听政”.例如：
不定式：It is difficult to study English Grammar.学习英语语法很难.
动名词：It is no good eating more.多吃食没有好处的.
从 句：It's a pity that he couldn't visit Mary.
二、I could drive a car by the time I was sixteen 中的by the time I was sixtteen 可以作如下几种理
1.最易于理解的是把 by the time 看成一个复合的引导时间状语的连词,如果这样,它就是时间状语从句；
2.比较复杂的办法则是把 by the time 和 I was sixtteen 分开来分析：
* by the time 介词短语作时间状语,意思是“到那时为止” 或者“在那以前”；
* I was sixtteen 是省略关系词 when 或 at which 的定语从句,即“（在）我16岁的时候（以前）”.
英语语法里常把“the + 表示时间的名词”规定为复合连词,用来引导时间状语从句,如：
the moment 就在...那一刻
the time 就在...那个时刻
the day 就在...那一天
the year 就在...那一年
g

第一个是it作形式主语，实际应该是：That he couldn't visit Mary is a pity.
第二个是when时间状语从句,省略了when. (I could drive a car by the time when I was sixteen.)

第一个不是定语从句 且是个错误的句子 因为 他不能见玛丽 这是遗憾的内容 所以这句话应是一个同位语从句 而在同位语从句中 that是不能省去的
应该是 It's a pity that he couldn't visit Mary.
这样就是一个同位语从句
第二句是一个时间状语从句

时间状语从句。