例句：
1. He spends his spare time visiting schools to warn pupils off drugs.
他用自己的業(yè)余時(shí)間訪問(wèn)學(xué)校，告誡小學(xué)生遠(yuǎn)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游客們熄滅香煙。
3.I must warn you against raising your hopes.
我必須警告你不要期望過(guò)高。