Answer:
I took the dog for a long walk. - simple
The dog ran happily through the park because he had been stuck in the house all day. - complex
He stopped to smell several flowers, and he chased a rabbit. - compound
Step-by-step explanation:
A simple sentence is constituted of a only one independent clause. It has a subject and a predicate, and it expresses a complete thought. That is what we have in "I took the dog for a long walk."
A complex sentence is formed by joining two clauses, one independent and the other dependent/subordinate. The dependent clause needs the independent one to make sense, since it does not express a complete thought on its own. That is what we have in "The dog ran happily through the park because he had been stuck in the house all day." The subordinating conjunction "because" introduces the dependent clause.
A compound sentence is formed by joining two independent clauses connected by a coordinating conjunction - for, and, nor, but, or, yet, so. That is what happens in "He stopped to smell several flowers, and he chased a rabbit." In this case, the conjunction is "and".