This new airport has been in service for six years

问题补充：

This new airport has been in service for six years ago错在哪里可是通过since引导后似乎ago还是可以出现在完成时的

答案：

去掉ago.

This new airport has been in service for six years./This new airport has been in service since six years ago.

两种改法Both OK.

已更新!