Please consult a German grammar book. This is basic grammar.
This is a declarative sentence. In German, declarative sentences have the core of their predicate verb (the verb part which is conjugated) in second position in the row of items. That's one of the very few absolute word order rules in German.
Hunde schwimmen wirklich gerne.
Die Hunde schwimmen wirklich gerne.
Die Hunde sind wirklich gerne geschwommen.
Both Hunde and die Hunde are the first item in their sentences. In the last sentence, sind is the core of the predicate verb (the Perfekt auxiliary sein in 3rd person plural), and geschwommen is the Partizip II of schwimmen, as needed for the Perfekt tense.
You can also switch Hunde and wirklich gerne and get a valid German sentence:
Wirklich gerne schwimmen Hunde.
What's the difference? In this sentence, the item wirklich gerne, an adverbial of manner, is put in front, which makes it the most emphasized item of the sentence. The topic.