grammar - In the sentence "The wallet has fallen." Is the verb "fallen" being used in the passive form and is it grammatically correct?

I feel like it is grammatically correct, but as a native speaker I am aware that I will say and write things that are not "by the book" when it comes to grammar.

I also know that you can have a sentence that is in a prepositional-passive tense such as "This roof has been fallen from by many people" but I don't think that's this unless the unspoken "onto the ground" or other surface counts towards the correctness of the sentence.

Answer

"The wallet has fallen" is a perfectly grammatical, normal, English sentence.

It is not passive: "fall" is an intransitive verb, and does not normally have a passive.

Passives in English are formed with the Past Participle (which "fallen" is) and parts of the verb "be" (or, in colloquial speech, "get").

"This roof has been fallen from" is passive, but it's an odd sort of passive because "the roof" is not the direct object of the corresponding active "Z has fallen from the roof". It is grammatical in current English, however.