Many in society expect longer sentences for most offences,while the experts tell us that,generally speaking,custodial sentences,except in special circumstances,do more harm than good and that short sentences probably do less harm than long sentences.

　　要点：这是一个主从复合句。while在这里不是表示时间，意思是而。tell的直接宾语是两个由that引导的从句：that custodial sentences do more harm than good和that short sentences probably do less harm than long sentences。except in special circumstances本事第一个that从句的状语，但因为过长，所以插在主语和谓语之间。再者，这两个从句中的比较结构是完全不同的：harm和good都是do的宾语，是在宾语中的比较，而than long sentences是两个句子之间比较，long sentences后省略的谓语do(harm)。

　　译文：社会上许多人期盼着对大多数罪行判处较长时间的刑期，不过，专家们告诉我们，一般的说，特殊情况外，判处监禁的弊大于利，短期监禁的危害性可能小于长期监禁。