since的使用方法

老师你好，在经济学人看到一篇题目为：‘Time to end the academic arms race’的文章，其中有一点中有如此一句话：“As more young people seek degrees, the returns both to them and to governments are lower. Employers demand degrees for jobs that never required them in the past andhave not become more demanding since.”请问后半句have not become more demanding since这句话如何理解？其中since的使用方法是什么呢？谢谢你！

Employers demand degrees for jobs that never required them in the past and have not become more demanding since.
【翻译】雇主们目前需要拥有学位的工作在过去从来没需要过，此后也没变得更苛刻。

since 可以用作介词、连词或副词。单独用就是副词，后面不接宾语时，看作省略了已知的宾语（ then），即：since then。又如：

He left home two weeks ago and we havent heard from him since.
他两星期以前离开了家，到目前大家一直没他的消息。