since引导时间状语从句

since 21 century ago-----since nine years ago

我给你改啊，看着：Since 21st century ago,the computer has been widely used for personal daily life./Since 21st century ago,the computer is widely used for personal daily life.可以是现在完成时，也可是一般现在时，因为现在完成时是现在与过去联系在一起的时态，主要强调现在的情况，所以两种时态都可以。例如，It is /has been three years since I come here.
怎么样？够明白了吧!好了，祝你进步哟！

原句有错。
正确的讲法是：Since 21st century ,the computers have been widely used for personal daily life.
since...不能与ago连用。since引导的从句一般用一般过去时。如：I have studied English since I was 7 years old.

widely used
完成进行时
since ago之间用一个时间段是可以的比如since 3 years ago

状语从句

状语从句主要用来修饰主句或主句的谓语。一般可分为九大类，分别表示时间、
地点、原因、目的、结果、条件、让步、比较和方式。尽管种类较多，但由于
状语从句与汉语结构和用法相似，所以理解和掌握它并不难。状语从句的关键
是要掌握引导不同状语从句的常用连接词和特殊的连接词即考点。现分别列举

时间状语从句

引导词：when, as, while, as soon as, while, before, after, (since),
till, until, the minute, the moment, the second, every time, the day，
the instant, i