has been since是什么时态
has been since是现在完成时态。
Since 1977 otter hunting has been illegal.
捕猎水獭1977年以后就被列为非法了。
Since 1908, chlorine has been added to drinking water.
自从1908年起，氯就被添加到饮用水当中了。
only since 1962 has the show been held biannually.
从1962以后，这个节目才开始每年举行两次。
She has been in the playoffs every year since she made the varsity.
自从加入校队后，她每年都参加季后赛。
Polling has been heavy since 8 a.m.
从早上8点以来投票很踊跃。