关于非谓语动词的问题judging from(by) his appearance,he must be an acto

1.Judge by (from):make a judgement on the basis of ...根据〔由〕…作出判断
judging by 一般只作插入语,放在句子开头,表明说话者的判断依据,与具体的人或物无关.
e.g.Judging by the sound of his voice,the man couldn't be very young.
听声音,那人已不太年轻.
又如：Frankly speaking,he is incapable of doing this kind of work.
坦白地说,他没能力做这项工作.
这里前面的分句仅仅是插入语,和后面没用逻辑关系.不能改成 Frankly spoken,he is...
2.若确实表达被动意义,也可以用judged by
e.g.This case is being judged by the Supreme Court.本案正由最高法院审理.
A man should be judged by his deeds,not his words.判断一个人应根据他的行为,而不是他的言词.

呵呵，看过楼上两位的回答之后，我想还是让我来稍稍说明一下吧！
非谓语动词的语态是使用主动还是被动，单单从与主句主语的关系上来判断，这是不够准确的，只是初级的水平。真正判断的方法是“根据句意义”或“根据语者的逻辑”。就这句话来说。
judging from(by) his appearance, he must be an actor.
这句话是谁说的？你说的。是你judg...

judging from或to judge from(by) 在句子中是插入语，固定用法。此时不能根据主句的主动被动关系，来决定动词形式。
若是judging 单独不构成插入语，这时就要按照非谓语动词判断是否合意：judging his appearance, he must be an actor。judge 是及物动词，与主句主语是主动关系，看句意是否合适，显然不成立。
一些...

judging from是固定用语，根据……判断”，用作独立结构，不用考虑与逻辑主语的一致性，不存在judged from用法，类似的还有generally speaking, frankly speaking等等