1. in the sense of apprise
- alert
- notify
- give notice
- apprise
- tell
- inform
- make someone aware
- let someone know
- give fair warning to
- give a warning to
- put someone on notice/guard
- forewarn
- raise/sound the alarm
- remind
- put wise
- tip off
2. in the sense of urge
- exhort
- advise
- counsel
- urge
- put on the alert
- caution
- put someone on guard
Example Sentences of Warn
1. I need to warn you urgently.
2. They warn travelers about potential dangers.
3. She warned him not to proceed.
4. Please warn me if anything changes.