The man,___in thought there,is our english teacher.为什么用lost而不是losting

跟楼上的说得一样，be lost in thought是陷入沉思的意思
没有losting. losing是动名词，但是不能形容什么东西

lost的其中一个用法就是：
giving so much attention to what you are doing that you are not aware of anything else that is happening around you
比如：Ann was completely lost in her book.

跟lost in thought一样，可以lost in books, lost in the song........
但是就不能用losing

be lost in thought是陷入沉思之中，但因为在句中不作谓语，所以be被省略。这里lost in thought是过去分词短语作伴随状语。

lose是及物动词，那么losing是主动形式，后面需要有宾语，可是这里没有，所以不能用losing。

Losing the bike made him depressed.