Grammar Focus: Articles: Definite, Indefinite, Zero

• Indefinite (a / an) — before singular countable nouns that are general/unspecific. a before a consonant sound, an before a vowel sound: a mosque, an hour.
• Definite (the) — before specific nouns, unique things, geographical features (rivers, oceans), and superlatives: the Yamuna River, the best example.
• Zero (no article) — plural/uncountable nouns in general, and most proper nouns (people, cities, countries, languages): Shah Jahan, Agra, India, Muslim art.