I'm absolutely not qualified to address this definitively, but my understanding is that Hiragana is typically used for verb conjugations, adjective inflections, particles (like "の" for indication of possession for example), some natively Japanese words where no one really uses the Kanji for whatever reason, and some other stuff that's similar in nature. Katakana is typically used for writing foreign words so that they use the pronunciation rules of Japanese (like "コカ・コーラ" for Coca-Cola for example) and onomatopoeias.