例句：
1. He spends his spare time visiting schools to warn pupils off drugs.
他用自己的`业余时间访问学校，告诫小学生远离毒品。
2. Signs across the entrances warn all visitors to stub out their cigarettes.
入口处的告示牌提醒游客们熄灭香烟。
3.I must warn you against raising your hopes.
我必须警告你不要期望过高。