be worth doing~相知之类的词不用被动语态，为什么？？？还有那些词不能用被动语态？

一。常见的系动词look （看起来），sound (听起来)，taste （尝起来），smell （闻起来），feel（感到），go （变得），grow (变得)等。

二、一些不能接宾语的动词短语也没有被动语态。
1．The war broke out in 1937.（break out,爆发)
2．The story took place in a small mountain village.(发生，take place)

三、甚至有些及物动词和可以接宾语的动词短语要看他们作什么意思讲，有时也只能有主动语态而不宜于用被动语态。
1．We have six classes every day. (have,上课)
2．The hall can hold more than 500 people. (held,容纳)
3．The project lasted 14 years and cost I billion US dollars.(last,延续)
4．No dish suits all tastes. (suit,适合)
类似的还有fit ,catch ,get ,take ,own ,meet 等。

四、而有些动词和短语兼有及物和不及物两种用法，所以前者有被动语态，而后者没有。
1．He serves in the Navy Department. (服役，不及物)
2．Hamburger is served in this restaurant. (供应，及物)
3．The plane toke off at eight last night. (起飞，不及物)
类似的还有look into （作往里看，不及物；作调查、研究，及物） look up (作好转，不及物；作查阅，及物)等。

五、而如果及物动词后的宾语是反身代词，相互代词；或者宾语前面有和主语同一人物的物主代词；或者是同源宾语的动词，也常常不能转换为被动语态。例如：
1．The thief hid himself behind the door . (宾语为反身代词)
2．We should learn from each other. (宾语为相连代词)

六、带宾语从句的句子常常没有被动语态
1．Marx found that his English was too limited.
2．I don’t know where he lives.
但是，如果主句的谓语动词是believe ,say ,know ,report 等，且宾语从句是由that
or
whether引起的，则常可以用It is believed (say ,know ,report) that (whether)的句型。

七、有些及物动词有其习惯性用法，常用主动代替被动：
1．It is a pretty material ,but it doesn’t wash.
比较：My shirt is usually washed by myself.
2.
The new Ford is selling badly.
比较：All newspapers have been sold out.
类似的还有clean ,lock ,write ,play ,start ,cut等。

八、最后要说的是另一种情况：英语中有不少动词及动词短语常用被动形式来表示主动的含义。
1．
Be seated, please!
2．
We must be prepared for the worst.
类似的还有be determined to ,be absorbed in , be gone , be married to , be hidden , be interested in
……and
advanced mathematics ,experienced school , learned man 等中这些定语用的过去分词也是用过去分词表主动

worth不是动词，是形容词，它固定后面只跟doing或to be done. 这些单词要分别记，见一个记一个