There are a number of preposition words that change the meaning of a phrase when used with a particular word. For example, you can use Warn with different prepositions, and you will get a different meaning every time. You can see the examples below:
1. Warn against
Warn against (an action)
Example: I have already warned you, against your such negligence.
2. Warn of
Warn of (danger)
Example: I have already warned you of the risk involved.