1、My brother has two sons,both of whom are working hard .
both of +whom; 介词 +whom； 介词+ which.
2、I'll take the conditions into careful consideration which you have attached to this contract.
从句成份不完整.很显然动词attach 后面缺少宾语,attach ...to
3、It is the first time that my cousin has gone abroad.
这是一个固定句型.
4、It was 2010 when he traveled in France.
不是强调句 .因为强调句qudiao it was that 仍然为一个完整的句子.你去掉之后,会发现 2010前面少了一个介词in．
5、it was in the garden of his old house where he grow up that he dug up a pot of gold.
你将句子简单化,变成简单句子就是 it was in the garden of his old house that he dug up a pot of gold.这就是一个强调句.这个短语中有了定语从句.in the garden of his old house where he grow up.
6、when people talk about the famous scenic spot in Anhui.the first that comes into mind is the Yellow Mountain.
填one的时候,主语变成了 the first one comes into mind is the .那么我们知道主语从句放在主语的位置上要加上 that .That the first onecomes into mind is the .就可以；原句是一个定语从句.that的关系代词,the first (famous scenic spot ) ,其实是省略了括号里的.补充上来就好理解了.
楼主,希望能解开你的谜团.