The lazy dog is sleeping soundly on the couch.

Meaning: This sentence is similar to the first sentence, but it adds the word "the" to specify that we are talking about a particular dog. It also uses the verb "is" to describe the state of the dog, which is sleeping.

Choice of Words: The is used to specify a particular dog. Is is a verb that means to exist or to be in a certain state. Sleeping is a present participle that describes the action of the dog.