“any girl or boy who has not reached adulthood, for whom the street (in the broadest sense of the word, including unoccupied dwellings, wasteland, etc.) has become her or his habitual abode and/or sources of livelihood, and who is inadequately protected, supervised or directed by responsible adults”
The use of a, an, or the is a signal that a noun is coming.
In the English language, only nouns are preceded by articles. A is used when a noun starts with a consonant (a book, a table, a bottle); an is used when the noun starts with a vowel (an apple, an umbrella, an avocado); the is used when you are referring to a particular thing (The boy I just met is called Mark.)
Auggie has had 27 surgeries since he was born.