原文中有几处问题,先改正,再翻译：
Hello,Chen,
I am very sorry that I didn't reply you in time .【As 大写】for the last question you ask me how to instruct your students ,it is a good question.【 But 大写】the 【problem单数】is that I am too busy these days.【As大写】 for the roots of the words ,we have suffix that 【means】 【adding 】the 【ending 】【to】the words to change the meaning【of】 【the words】.【 It 】is very important to know about it .【It】 can enlarge your vocabulary.【Good 】luck !
陈（先）生!
很抱歉没有及时回复你.你的最后一个问题——怎样指导学生,真是个好问题/问得好.不过,（没有及时回复的）问题是我近来很繁忙.关于词根,我们有后缀,其意思是给词加上词尾,以改变词意.了解这一点很重要,他可以扩大你的词汇量.