例句：
1. He spends his spare time visiting schools to warn pupils off drugs.
他用自己的`業餘時間訪問學校，告誡小學生遠離毒品。
2. Signs across the entrances warn all visitors to stub out their cigarettes.
入口處的告示牌提醒遊客們熄滅香菸。
3.I must warn you against raising your hopes.
我必須警告你不要期望過高。