since 引导的时间状语是动作的开始时间，如:
I have studied English since 1990.
而by引导的时间状语通常是动作的结束时间，如：
I had learned 25 English songs by the end of last term.

从属连词语法分类：

种类	语法作用	连词举例
从属连词	引导原因状语从句	Because, since
	引导条件状语从句	If, unless, as long as
	引导目的状语从句	So that
	引导结果状语从句	So…that
	引导方式状语从句	As, just as, as if
	引导让步状语从句	Though, although, even though, no matter what, however, no matter how, whoever, no matter who, wherever, no matter where
	引导比较状语从句	As…as, not so…as, than, less…than, the more…the more
	引导名词性从句	Who, whom, whose, which, that, when, where, why