例句：
1. He spends his spare time visiting schools to warn pupils off drugs.
他用自己的業余時間訪問學校，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高。