首先分析句子成分
his english has much improved为主句,时态是现在完成时,故由此寻找引导时间状语从句的选项,
since 符合时态要求
故为D
而其余三项不可引导完成时态的状语从句
考点为现在完成时其他三项时态应是什么样的？你好before，after，when引导的时态最常见的为一般过去式，Annie was born a few weeks before Christmas.安妮在圣诞节前几个礼拜出生。After breakfast Amy took a taxi to the station.早饭以后，埃米乘出租车去了车站。when的时态太广泛其实，该阶段考查范围比较窄，都是有重点的做好老师的讲义就可以了