several years做主语时,谓语动词用单数还是复数?为什么?

several years 要接复数动词,
比如
several years have passed since I began playing the piano.
虽然我们说
10 years is a long time to wait
但我们说
10 years have passed since I started studying English
前一句 10 years 是一个整体
后一句 10 years 是指复数的 10 个年
十个年,十个月,十个星期,数年,等等 是复数

用单数，这时可以将several years做主语看成一个整体