•\x05when it comes to sth中的 it到底是什么成分?指的是什么呢?

是真正主语.When it comes to German,I know nothing,这个句子可以改写成为：When we / you / people talk about German,I know nothing.
也就是说,when it comes to something=== when we / you talk about something,
when the topic is about something
看例句：When the topic is about travelling,where have you been in China?
=== When it (it== the topic) comes to travelling,where have you been in China?
所以,it===the topic.(主题).when it comes to sth的翻译是：在谈起 -- - -时,-- - - -.

上句的意思大概是：说到德语，我什么都不懂！其中的it是形式主语，没有实际意义，是为了句子的结构而用在里面的。

固定句型，当然是主语，无实际含义

it只是一个形式主语，没有实际的含义
when it comes to sth是一个固定搭配，意思是当提到。。。。的时候