我的答案是：write a letter to sb.或者write sb.a letter。这是write跟双宾语的用法。a letter是直接宾语，sb.是间接宾语。直接宾语在前时后面跟介词to。类似结构的有：give sb.sth.=give sth. to sb.，lend sb.sth=lend sth.to sb.buy sb.sth.=buy sth.for sb.show sb.sth.=show sth.to sb.