Answer:
1. A dog barked loudly outside my window last night.
2. An apple fell from the tree and landed on the ground with a thud.
3. A new restaurant opened up in our neighborhood, and I'm excited to try it.
4. An umbrella is essential on a rainy day to stay dry.
5. A cat sat on the windowsill, watching the birds outside.
Explanation:
"A" and "an" are classified as indefinite articles in the English language. They are used before nouns to indicate that the noun is non-specific. "A" is used before words that begin with a consonant sound, while "an" is used before words that begin with a vowel sound.