“However,” 在这里只是一个引导性的副词，这个句子就是有意义了。

Once it's clear that "however" is just an introductory adverb, the sentence makes sense.

youdao

句子主干是it will pass through sharp metal bars。 第一个which引导的定语从句修饰的是sharp metal bars。

First it will pass through sharp metal bars which will tear open the plastic bags in which rubbish is usually packed.

youdao

从句不是一个句子，通常由引导词引导。

A dependent clause cannot be a sentence. Often a dependent clause is marked by a dependent marker word.

youdao