Grammatical Analysis
Articles: Use of THE - A or AN
Example:
Using a very long ladder he climbed to the roof, to sneak in by the chimney.
If you say a roof you demonstrate that you do not know that you're talking about specific roof. But when you say THE roof, you are talking about a specific roof.
conclusion: for talk about something in general you say..... THE (house, flower, car, etc.)
for talk about something in particular you say.....A (house, flower, car, etc.)
why you say AN APPLE?
Because the first sound of the noun is a vowel.
A dog 
An elephant
Subject and Object pronouns
subject pronoun object pronoun
I ME
YOU YOU
HE HIM
SHE HER
IT IT
WE US
YOU YOU
THEY THEM
Example:
In the heart of the forest lived three little pigs who were brothers. The wolf always was chasing them to eat them.
them = is used to refer to The three little pigs (they)
The wolf stood to think over the house, searching for some place for him to enter.
Him = is used to refer to The wolf (he)