Here's why:
* Prepositions show the relationship between a noun or pronoun and another word in the sentence. They often indicate location, time, or direction.
* For typically expresses purpose, duration, or benefit:
* "I bought a gift for my friend." (benefit)
* "We waited for an hour." (duration)
* "He built a house for his family." (purpose)
* Since indicates a point in time from which something has continued:
* "I've been working here since 2020."
* "It has been raining since yesterday."
Let me know if you'd like more examples or have other grammar questions!