The sentence is grammatically incorrect.
The correct sentence should be: "It is hard for my little brother to recall what he has done."
Here's why the original sentence doesn't work and the breakdown of the correct sentence:
* "It is hard": This is the subject and verb, expressing the main idea of the sentence.
* "for my little brother": This is a prepositional phrase functioning as an adverbial phrase, modifying "hard" by explaining who finds it hard.
* "to recall": This is an infinitive phrase functioning as the object of the verb "is."
* "what he has done": This is a noun clause functioning as the object of the infinitive "to recall."
"Usually" and "by" are not present in this sentence. They are not needed for the sentence to be grammatically correct and make sense.
Let's look at how "usually" and "by" might be used in similar sentences:
* Usually: "Usually, it is hard for my little brother to recall what he has done." ("Usually" modifies the entire sentence, acting as an adverb.)
* By: "My little brother is usually helped by me to recall what he has done." ("By" introduces a prepositional phrase that tells us who helps him.)
In summary: The original sentence was grammatically incorrect. The correct sentence doesn't need "usually" or "by." We can use those words in different sentences to show how they function as adverbs and prepositions respectively.