6. 條件狀語從句：
連線詞：if, once, as long as, on condition that
① 如果引導的條件可以實現，主將從現；
② 如果引導的條件和事實相反，則用虛擬語氣；
eg：If winter comes can spring be far behind.
As long as it doesn't rain, we can play.
You can go out on condition that you wear an overcoat.
Modern cars are far tougher to steal, as their engine management computer won't allow them to start unless they receive a unique ID code beamed out by the ignition (點火) key.
