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 prepare with different prepositions, and you will get a different meaning every time. You can see the examples below:
1. Prepare for
Prepare for (be ready)
Example: I am preparing for the IAS exam.
2. Prepare against
Prepare against (danger)
Example: We must prepare ourselves against the danger of water pollution.