If "in" is used as a preposition:
* "in" is invariable and stays the same in all tenses. It will still be "dans" in the past tense.
Example:
* J'étais dans la cuisine. (I was in the kitchen.)
If "in" is part of a verb:
* You need to conjugate the verb in the past tense. The most common past tense in French is the Passé Composé.
Examples:
* Entrer (to enter) --> Je suis entré (I entered)
* Sortir (to go out) --> Elle est sortie (She went out)
Let me know if you have a specific sentence in mind, and I can help you find the correct past tense form!