there have been 的用法

there be句型是倒装句,意思是"有",be的形式由be后第一个名词的单复数决定；
there have been,意思是"曾经有",是there be句型的现在完成时态形式,此时be后主语为复数名词,如果be后主语为单数名词,则应为there has been
如
There have been some encouraging signs over the past couple of months.
在过去的几个月里有一些令人鼓舞的迹象.
There has been a singer playing guitar at the bar for many months.
曾经有个歌手在酒吧里弹了几个月吉他.