Mnemonic I learned in my high school Latin class, used to remember which prepositions in Latin take the ablative case:

Ab cum de, ex in pro, sine sub!

All other prepositions take the accusative case. Additionally, the noun following in (meaning, unsurprisingly, "in") takes the accusative case when expressing the goal or object of the action, but takes the ablative case when expressing the location of the action. In other words, in + acc. = "into"; in + abl. = "inside".