1. Dans: This is the most common and versatile way to say "in." It's used for places, times, and situations.
* Dans la maison: In the house
* Dans trois heures: In three hours
* Dans le doute: In doubt
2. En: This is used for places, modes of transportation, and sometimes for time periods.
* En France: In France
* En voiture: In a car
* En un an: In a year
3. À: This is used for specific locations and sometimes for times.
* À l'école: At school
* À Paris: In Paris
* À midi: At noon
4. Sur: This is used for surfaces, positions, and sometimes for time periods.
* Sur la table: On the table
* Sur le toit: On the roof
* Sur cinq ans: Over five years
Remember that the best choice will depend on the specific context of your sentence.