Choose appropriate articles for the given sentence:
I saw _____ accident this afternoon where_____ bus had crashed into a tree.

In the given sentence use 'an, a' will come in the place of blanks.
The article 'A' is an indefinite article that is used with a consonant sound of a singular noun.
Example: a boy, a car, a helicopter.
The article 'AN' is an indefinite article that is used with a vowel sound (a, e, i, o, u) of a singular noun
Example: an elephant, an itchy sweater, an ugly duck.
Hence the correct sentence is - I saw an accident this afternoon where a bus had crashed into a tree.