The is the sentence I'm uncertain about:

These old journals are wonderful to learn what life was like a century ago.
I know the infinitive is 'to learn.' I think it is being used as an adverb. It's modifying the adjective 'wonderful.' It's not a noun used as a direct object or a noun as a subject. Could it be an adjective because it's modifying journals? These types of sentences confuse me. I'd be grateful for any help, thank you.

That's a bad sentence. Are journals able "to learn" anything?

Better sentence: These old journals are wonderful for us to read and learn what life was like a century ago.

If you were given that badly phrased sentence in a text or by your teacher, you need to ask for better educational resources.

Yes, the sentences we receive are really bad! I've talked to this teacher about this. You know how there are some who feel they are above question? She's one of them. With that in mind, do you concur it's an adverb clause? Thanks so much!

Yes, you're stuck with this!

Yes, "to learn" is an infinitive, being used here as an adverb modifying "wonderful."

Thank you Writeacher, I pray you are blessed for your help.

You're welcome ... and thank you, too!

I can help explain the role of the phrase "to learn" in the sentence you provided.

In this case, the phrase "to learn" functions as an infinitive phrase and acts as an adjective modifier for the noun "journals." Infinitives are verb forms that are generally preceded by the word "to" and can function as various parts of speech depending on their usage.

In the sentence, the phrase "to learn" answers the purpose of the journals: they are wonderful for the purpose of learning what life was like a century ago. The word "wonderful" is an adjective describing the journals, and "to learn" further describes the quality or characteristic of the journals.

To identify the phrase, you correctly recognized it as an infinitive because it starts with the word "to." Regarding its function, it acts as an adjective because it modifies the noun "journals" by specifying the purpose or quality of the journals.

To summarize, in the sentence "These old journals are wonderful to learn what life was like a century ago," the infinitive phrase "to learn" serves as an adjective and modifies the noun "journals." It describes the purpose or quality of the journals in allowing one to learn about life a century ago.