The indefinite articles "a" and "an" can be used in the sentences as per these rules.
a. To denote an indefinite person or thing.
b. To generalize, a proper noun or to mean one like it.
c. In a few phrases to denote similarity.
d. To ensure the individuality or to mean each.
e. Before words denoting collective principles.
f. In numerical sense.
g. Before 'few', 'little', etc. to convey the sense.
h. To change an abstract noun in to a common noun.
i. Before fractional term.
a. A boy in the class beat her.
b. I never saw a man as able as Newton.
c. Birds of a feather flock together.
d. He earns 20000 rupees a month.
e. He want a dozen apples.
f. Seven days make a week.
g. I am in need of a few books.
h. He did me a favour.
i. He gave her a fourth part of the pear.