Jim had no choice except to resign from the position.

to resign - infinitive
used as an adverb?

It's an adverb. But it's used as a noun, the object of the preposition "except."

In the given sentence, "to resign" is not used as an adverb. It is actually part of the infinitive phrase "to resign" acting as an adverbial phrase modifying the verb "had." The word "resign" is the base form of the verb, and "to" is the infinitive marker.

To find out if "to resign" can also be used as an adverb, we can look for examples where it modifies a verb, adjective, or another adverb. For instance:

1. He decided to resign immediately. (modifying the verb "decided")
2. She seemed eager to resign. (modifying the adjective "eager")
3. They spoke too softly to hear, but I was close enough to resign witness to their conversation. (modifying the adverb "softly")

In these examples, "to resign" functions as an adverbial phrase, providing additional information about the verb, adjective, or adverb it modifies.