when引導的從句是什麼從句?

　　When is the best time to ring New York?

　　什麼時間給紐約打電話最好？

　　It was a difficult situation, when you think about it.

　　仔細想來，當時處境是很困難的.。

　　When they set out they were well prepared.

　　他們出發時有很充分的準備。

　　Text me when you're on your way.

　　路上給我發簡訊吧。

　　When do the new regulations come into force?

　　新規章什麼時候開始執行？