ago是指从这时算的,before指从过去的某一时刻向前推
after是指在.之后until与TILL差不多until和till两者都可作介词、连词,一般情况下可以互换使用.用于肯定句时,主句的动词只用延续性的,它所表示的动作一直延续到till或until表示的时间为止,意为“直到……为止”；用于否定句时,主句的动词一般是非延续性的,也可以是延续性的,它所表示的动作直到till或until所表示的时间才发生,意为“直到……（才）”.如：
She watched TV until / till her mother came back.
她看电视直到她母亲回来.（看电视的动作延续到母亲回来才结束）
She didn”t watch TV until / till her mother came back.
直到她母亲回来她才（开始）看电视.（看电视的动作直到她母亲回来才发生）
为了同学们更为系统地掌握它们的用法,现将一些相关知识点作具体的介绍：
1.until和till可以作介词或连词.作介词用时,后面接名词或副词；作连词用时,常用来引导一个时间状语从句.当它们用于肯定句中,主句动词常为延续性动词,表示这个动作一直延续到till / until所表示的时间为止.当它们引导时间状语从句时往往要用一般现在时代替一般将来时.如：
They worked until / till six o”clock.
他们一直工作到六点钟.
He will wait for you until / till your mother comes.
他将等你到你母亲来.
2.not until / till意为“直到……才”,主句常用终止性动词（即非延续性）,表示until /till所表示的时间一到,该动作就发生.如：
He didn”t go to bed until / till eleven last night.
昨天夜里他直到十一点才上床睡觉.
She didn”t know me until / till last week.
她直到上一周才认识我.
Einstein was not able to return to his homeland until the war was over.
直到战争结束后爱因斯坦才得以回国.
注意：not until放在句首时要用倒装语序,如上面第三个例句可以改为放在句首的倒装句：Not until the war was over was Einstein able to return to his homeland.
3.如果要放在句首,只能用until而不能用till,如：
误：Till I finished my homework,mother didn”t let me out.
正：Until I finished my homework,mother didn”t let me out.
直到我做完家庭作业妈妈才准我出去.
4.强调结构中也常常出现not until结构,常见句型为：It is / was not until…that.如下面这个句子：
She didn”t find me until I called her from behind.
直到我从后面叫她之后,她才发现了我.
改为强调结构：It was not until I called her from behind that she found me.
注：但在强调句型中,not until置于句首构成倒装句以及until放在句首时,均不能用till替换until.例如：
It was not until 1920 that regular radio broadcast began.
直到1920年才开始有定期的无线电广播.
Not until 12 o”clock last night did Tom come back.（倒装句）
昨晚直到十二点汤姆才回来.
Until I was 20 I had never been away from my hometown.
我二十岁以前,从未离开过家乡.
