One of the memorable moments of Hard Times occurs early in Book I,in chapter iii,when Mr Gradgrind,walking past Sleary's circus booth,discovers his children Louisa and Tom outside,striving to peep in.其中when Mr gradgrind,walking past Sleary's circus booth,在句中是什么成份，这是when的什么用法？请大神指教。

walking past Sleary's circus booth 是插入语，你把它挪到句末再看就明白了。