Is this sentence a simple, complex, or compound sentence? I believe that it is a complex sentence because it is not two sentences put together, is that right?

The teacher returned the homework since she was finished grading it.

correct.

The sentence "The teacher returned the homework since she was finished grading it" is a complex sentence. A complex sentence is a sentence that contains one independent clause and at least one dependent clause.

To determine if a sentence is complex, we need to identify the independent and dependent clauses.

In the given sentence:
- "The teacher returned the homework" is an independent clause because it can stand alone as a complete sentence.
- "since she was finished grading it" is a dependent clause because it cannot stand alone as a complete sentence; it depends on the independent clause to make sense.

Therefore, because the sentence has one independent clause and one dependent clause, it is classified as a complex sentence.