Analysis:
Given the sentence "When he is given a task, he works really hard", we need to determine the type of sentence structure being used.

Types of Sentence Structures:

Simple Sentence:
- A simple sentence consists of a single independent clause with a subject and a predicate.
- It expresses a complete thought and does not contain any dependent clauses.
- Example: "He works really hard."

Compound Sentence:
- A compound sentence consists of two or more independent clauses joined by a coordinating conjunction (e.g., and, but, or).
- Each clause in a compound sentence can stand alone as a separate sentence.
- Example: "He is given a task, and he works really hard."

Complex Sentence:
- A complex sentence consists of an independent clause and one or more dependent clauses.
- The dependent clause cannot stand alone as a complete sentence.
- Example: "When he is given a task, he works really hard."

Conclusion:
In the given sentence, "When he is given a task, he works really hard", the structure is a complex sentence. This is because it contains an independent clause "he works really hard" and a dependent clause "When he is given a task". The dependent clause "When he is given a task" cannot stand alone as a complete sentence, making it a complex sentence structure.