The definite article is used...
before singular and plural nouns when the noun is specific or particular
to refer to something that has already been mentioned or that is clear from the context or situation.
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The indefinite artice is used...
a boat a car a pencil
a uniform, a university, a European
an apple an elephant an insect
an hour
tree
umbrella
igloo
teacher
eskimo
house
boy
egg
ostrich
pear
icecream cone
duck
map
eagle
unicorn
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