[請益] 用逗點隔開的短句

[snob2 (ggg)]

   請問

       句子中出現有逗點或破折號隔開的短句
       其作用主要是什麼

       如看到一句

       Some officials threw extravagant banquets, which is a
       traditional Chinese indulgence, almost a disease, and a
       way of both entertaining and showing off -- all at the
       expense, and in the name, of the state, at a time when
       the government was extremely short of funds.

       第一個問題是上句的文法有沒有問題

       第二個問題是逗點和破折號的作用到底是什麼

       謝謝


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