Learn Icelandic: Dýrin og fuglarnir sofa núna. - The animals and the birds are sleeping now.

This is because they have different grammatical genders. Dýr (animal) is a neuter noun, and its plural form is also dýr. To make a plural neuter noun definite (meaning "the"), you add -in, making dýrin. Fugl (bird) is a masculine noun. Its plural is fuglar, and to make it definite, you add -nir, resulting in fuglarnir.

Is dýrin both the singular "the animal" and the plural "the animals"?

No, but they look very similar! "The animal" (singular) is dýrið (adding -ið to the base word dýr). "The animals" (plural) is dýrin (adding -in). Neuter nouns can be tricky because their base singular and plural forms are often exactly the same (dýr means both "animal" and "animals" before you add "the"). The definite endings help you tell them apart.

Why does the verb sofa end in -a here?

In Icelandic, when the subject is plural like "they" (the animals and the birds), the present tense verb usually looks exactly like the infinitive (dictionary) form. That is why we use sofa here instead of a singular form like sefur (sleeps).

Why is the verb sofa used instead of a continuous form for "are sleeping"?