Happy for:
* Expressing joy on someone else's behalf: "I'm happy for you and your new job."
* Feeling pleased about something good that has happened to someone else: "I'm happy for her that she won the competition."
Happy in:
* Describing a state of being or feeling content: "I'm happy in my relationship."
* Indicating a location where someone experiences happiness: "She's happy in the countryside."
Other options:
* Happy with: "I'm happy with the results of the project."
* Happy about: "I'm happy about the upcoming vacation."
In summary:
* Use "happy for" when you're pleased about something good that has happened to someone else.
* Use "happy in" when you're describing a feeling of contentment or a location associated with happiness.
The specific choice of preposition depends on the specific meaning you want to convey.