Used to specify a noun as something already mentioned or known.

Used before a noun to indicate a single but unspecified person or thing.

वह

English Usage: The dog is barking.

Hindi Usage: वह कुत्ता भौंक रहा है।

Used before a noun that begins with a vowel sound to indicate a single but unspecified person or thing.

एक

English Usage: An apple a day keeps the doctor away.

Hindi Usage: एक सेब रोज डॉक्टर को दूर रखता है।