Prepositions are words that show the relationship between a noun or pronoun and another word in the sentence.
Here are some examples of how "at" is used as a preposition:
* I am at the library. ("at" shows the location of the speaker)
* She arrived at 3 pm. ("at" shows the time of arrival)
* He was surprised at the news. ("at" shows the object of the surprise)
Let me know if you have any other questions about prepositions!