When he saw those pictures, he couldn't help thinking of those memorable days in his hometown. 

这句话可以把前面的从句改写成：Seeing those pictures （非谓语动词seeing 的逻辑主语是 he ）。

请问：这种转换（即由于某种原因，可以把when和he省略，动词同时变成现在分词）在英语或语言学上有没有专门的术语来描述呢？

【答】你问题的意思，是不是想要这样解答：

In order for the sentence to be concise, clauses can be transformed into non finite verb forms.（为了使句子简练，从句可以转化为非谓语动词形式。）

例如：

When he saw those pictures, he couldn't help thinking of those memorable days in his hometown. 

= Seeing those pictures, he couldn't help thinking of those memorable days in his hometown. 

分词简化从句，或分词构句

When he saw those pictures, he couldn't help thinking of those memorable days in his hometown. 

简化为”现在分词作状语结构“

Seeing those pictures, he couldn't help thinking of those memorable days in his hometown.

现在分词是作”何种状语“需要从分词动作和后面谓语动作动作逻辑语义去进行判断。然后加上正确连词，就可以变成从句作状语形式。

这句话可以把前面的从句改写成：Seeing those pictures （非谓语动词seeing 的逻辑主语是 he ）。

请问：这种转换（即由于某种原因，可以把when和he省略，动词同时变成现在分词）在英语或语言学上有没有专门的术语来描述呢？

解答如下：

以上去掉"连词+主语"，并把原来谓语动词改为“现在分词”形式的语言现象称作为“现在分词作状语”结构，现在分词作状语属于非谓语动词作状语其中的一种。(或者说无谓语动词分句做状语）

现在分词分为以下2种

(1)一般时 doing （分词所表示动作与谓语动词所表示动作同时或者几乎同时发生）

Hearing the news, the girl burst into tears.(时间）

Not knowing how to work out the difficult physics problem, he asked the teacher for help.（原因）

Hearing their teacher's voice, the pupils stopped talking at once.（时间）

(2)完成时:having done(分词所表示动作在谓语动词所表示动作之前发生）

Having finished his homework, the boy went to bed.

Having lived with the girl for 5 years, we all know her very well.

Having finished his homework the boy was allowed to watch TV play.

特别备注(1)：从句作状语，只有主句和从句的主语相同时候，方可把从句简化为一个“非谓语动词作状语结构”或者“简化为一个”无谓语动词的分句作状语结构“形式.

特别备注(2)：如果从句和主句的主语不同，则需要使用独立主格结构作状语形式。