Here's why:
* It modifies a verb: It describes how something happens, often indicating a consistent or unchanging pattern. For example, "The sun invariably rises in the east."
* It can modify an adjective or another adverb: "The weather was invariably pleasant during our trip."
* It cannot function as a noun, pronoun, adjective, or verb.
Let me know if you have any other words you'd like to identify!