Before deciding which of these two words to use, you’ll want to read this article. You may find that your earlier uses of this pair were correct!

Previous is an adjective that means “earlier,” as in A brilliant light appeared in the sky just as it had the previous night.

Prior also is an adjective that means “earlier,” as in The president’s proposal is much less problematic than his prior calls to action.

Previous and prior are synonyms and can be used interchangeably. There is a small but vital distinction before them, however: Previous has an adverb form, previously, that can mean “before,” such as The FBI had previously investigated the man now accused of terrorism. “Before” is similar but slightly different than “earlier” in meaning. Prior does not have an adverb form.

So, if you intend to mean “earlier” and use it as an adjective in sentence, you can go with either previous or prior (Previous is slightly more common.). If you mean “before,” go with previously.