What is Warn?
Warn is (verb) 1. to inform someone of a possible danger We were warned to boil all drinking water. Children are warned not to play on the frozen lake. The group was warned to look out for pickpockets. The guide warned us that there might be snakes in the ruins. 2. to inform someone that something is likely to happen The railway has warned that there will be a strike tomorrow. The weather forecast warned of storms in the English Channel. (NOTE: You warn someone of something, or that something may happen.) ?
source: Easier English, Student Dictionary Upper Intermediate Level