Learn Danish: Hunden har et langt øre.

In Danish, the definite article ("the") isn't a separate word before the noun. Instead, it gets attached to the end of the word. Hund means "dog", and adding -en to the end makes it hunden ("the dog").

Why do we use et for "an ear" instead of en?

Danish has two grammatical genders for nouns: en-words (common gender) and et-words (neuter gender). The word for ear is an et-word (et øre), while dog is an en-word (en hund). You just have to memorize which group a noun belongs to when you learn it.

Why is the word for "long" spelled langt instead of just lang?

Adjectives have to match the gender of the noun they describe. Because øre is an et-word, the adjective lang gets a -t added to the end, becoming langt. If you were describing the dog instead, it would just be lang (for example, en lang hund).

Dogs usually have two ears. How would I say "The dog has long ears"?

You would say: Hunden har lange ører. The plural of øre is ører. When an adjective describes a plural noun, it gets an -e at the end, so langt changes to lange.