warn of (someone or something)
1. To provide information in advance about some potential source of harm, danger, or trouble. Analysts have been warning of a likely downturn in the economy for months now, so this dip in the market shouldn't come as a surprise. Police are warning of a number of escaped inmates who are at large in the area.
2. To caution someone about something; to inform someone of the risks or dangers of something. In this usage, a noun or pronoun is used between "warn" and "of." My mom always warns me of the dangers of texting while driving. I wish someone has warned me of her kleptomania before we started dating.
warn someone of something
to advise someone that something bad is likely to happen. I wish you had warned us of what was going to happen. Please warn John of the heavy traffic he may run into.
To make someone aware in advance of some actual or potential harm, danger, or evil: The doctor warned them of the flu epidemic. The employees were warned of the company's impending bankruptcy.