He (write)a letter to his parents every two weeks

附带why

writes，首先主语he决定动词write应该使用第三人称单数形式或者是过去式，而后面的时间状语为every two weeks表示一般现在时“每两周”，所以用writes。

希望能够帮助您哦，谢谢！

应该是He writes a letter to his parents every two weeks。

这里是一般现在时，he是第三人称单数。