Undefinite articles like 'a' and 'an' precede singular nouns and refer to things that are unknown, while 'some' and 'any' precede plural nouns and refer to an unspecified quantity. 'A' is used before consonant sounds and 'an' is used before vowel sounds. 'Some' is used in affirmative sentences and 'any' is used in interrogative and negative sentences.

1. UNDEFINITE ARTICLES
Undefinite articles qualify a noun and they always preceed it.
In singular, they refer to a person, animal or thing unknown by the speaker.
We use:
a an(before a consonant) (before a vowel)
dog
house
ant
eagle
a
a
an
an

2. UNDEFINITE ARTICLES
In plural, they refer to a certain quantity (we don’t know the exact number).
We use:
some (in affirmative sentences) any
(in interrogative and
negative sentences)
friends are coming home tonight.Some
I haven’t got brothers or sisters.any
Are you eating grapes?any