Final answer:
A compound sentence must contain at least two independent clauses and zero dependent clauses. It's when we add at least one dependent clause that the sentence becomes compound-complex.
Step-by-step explanation:
A compound sentence consists of at least two independent clauses that can stand alone as sentences and are typically joined by a coordinating conjunction (for, and, nor, but, or, yet, so). There are no dependent clauses in a pure compound sentence.
However, when talking about compound-complex sentences, we are referring to sentences that contain at least two independent clauses and one or more dependent clauses. A dependent clause begins with a subordinating conjunction (like although, because, or since) and cannot stand alone as a complete sentence.
For example, 'The sun was setting, so I decided to go for a walk, although it was getting cold' is a compound-complex sentence. It includes two independent clauses ('The sun was setting,' 'I decided to go for a walk') and one dependent clause ('although it was getting cold').
Therefore, to answer the original question: a compound sentence must have at least two independent clauses and zero dependent clauses.