The German preposition, mit always takes the dative case.
Depending on the article of the object the possibilities are (1) for
masculine and neutral article: mit dem (2) for feminine: mit der.
Most of the time, the use of mit is very similar to the English use of with. Other than that, for special phrases you will have [...]
Simplified, the meaning of the German ‘in’ preposition changes according to the case.
In the dative case, which asks the question where? or in German, wo? and pertains to a space. The English equivalent would be: at or also in. Depending on the article of the object the possibilities are (1) for masculine and neutral articles: [...]