The answer to their usage relies on their roots. If the base word stems from a Germanic language, the proper prefix is “un-.” If it stems from a Latin word, the proper prefix is “in-.” Yes, it's a nice simple answer, but not the easy answer that English spellers were hoping for.

What is the prefix of in?

Quick Summary. The prefix in, which means “in, on, or not,” appears in numerous English vocabulary words, for example: inject, influx, and insane.

What is the rule for adding the prefix in?

Rule 1 explains that for words beginning with 'l', the prefix 'in' changes to 'il-'. Rule 2 explains that for words beginning with 'r', the prefix 'in-' changes to 'ir-'. Rule 3 explains that for words beginning with 'm' or 'p', the prefix 'in-' changes to 'im-'.