Mostly is used when talking about something of volume or something that occurs often. “There are mostly brick houses in my neighborhood.” “The soccer games are mostly on Saturday’s. Sometimes they are on Sunday.” Primarily is typically used to express order. Primarily established priority. “I primarily work as a teacher’s assistant, but I occasionally work as a substitute teacher.” Chiefly means first and foremost; overall. “Chiefly, my mood is mellow.” “My mood is mellow, overall.” The words are synonymous, but English speakers choose which to apply in a conversation according its context. However; when speaking with friends and family the words mostly, mainly, and primarily are commonly used.