He writes to his e-friend once a month.对once a month提问

问题描述:

He writes to his e-friend once a month.对once a month提问

How often does he write to his e-friend ?

how ofen

How often does he write to his e-friends?

How often does he write to his e-friend?

对频率提问用how often.
所以How often does he writes to his e-friend