The only sound to be heard was the twittering of the birds above us.
to be heard在这句里是什么成份？主语的一部分吗？讲一下这里的to be吧。 它怎么用to？

Somebody is responsible for the accident。
为什么用形容词resonsible。译文好像是“某人对这个事故付责”从译文来看，用形容词responsible感觉有点不对？这句话的成份是？

The only sound to be heard was the twittering of the birds above us.
to be heard 作sound的定语，修饰sound，但不是主语的一部分。
这里的一个固定用法：only to do/to be done只能这样搭配
举例；He is the only one to be late for school.


Somebody is responsible for the accident。
这句话没错。意思为“某人要对这次事故负责” be responsible for “对。。。负责”。
somebody是主语 is responsible for 谓语 the accident作宾
语 就这么简单。