1)It's not easy for him to see them as frequently as before.
2)It has been 2 months since my younger sister joined the reading group.
My sister has joined the reading group for two months.
My sister has joined the reading group since two months ago.
3)That shop has been put into use for three years
4)How long is it since your friend left Beijing.
同学,注意掌握现在完成时喔