[思路分析]
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）
具体区别解答如下：
[解题过程]
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.
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.
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.