1. He took the dog to the veterinarian. Declarative. The speaker is asserting a known fact.
2. Will the girls meet us there? Interrogative. The speaker is asking a question. Also note the question mark.
3. Come with us. Imperative. The speaker is giving an order.
4. I can't believe he did that! Exclamatory. The speaker is expressing surprise or concern. Also note the exclamation mark.
5. I understood why he wanted to be there, and I certainly couldn't be upset at him. Compound. This sentence has two independent clauses joined by coordinator, in this case, a comma and the conjunction "and". It expresses two ideas: "I understood why he wanted to be here" and "I certainly couldn't be upset at him.
6. Sherry is involved in the drama club, soccer, and basketball. Simple. This sentence is just a single independent clause formed by a subject and a predicate. It only expresses one complete thought.
7. The dog waited excitedly to go outside, but the main paid no attention. Compound. This sentence has two independent clauses joined by a comma and the conjunction "but". It expresses two ideas: "The dog waited excitedly to go outside" and "the main paid no attention"
8. If John is able to come with us, we can probably make the trip in less than one day. Complex. This sentence contains a independent clause - that could stand as in independent sentence - and a dependent clause which cannot do it. The dependent clause is "If John is able to come with us" because we need more information about the complete thought it wants to express.