if my fridend had not come to help me.i would have

1.My Best Friend
Hello,everyone!My name is Xing Xiaoqing.You can call me Cherry.
I’m in Class 0506.Li Nan is my best friend.She’s thirteen years old.Her birthday is on October 3rd and she was born in Shen Zhen.She lives with her family in Haimen.She’s 1.60 metres tall.She has black hair and she wears glasses.She is very lovely and pretty.
She likes eating dumplings,chocolates,peanuts and vegetables.She dislikes pork,sweets and green tea.
She loves listening to music.She has lots of CDs.She likes reading and writing.She is the newest member of the Maths Club.She has many hobbies,such as dancing,running and playing computer games.Her favourite hobby is drawing,because she likes all the different colours.It is fun to look at the pictures when she finishes drawing them.The Dragon Boat Festival is her favourite day.She wants to be a doctor when she grows up.
We always have fun.We sit under the trees at lunchtime and have a good chat.Sometimes,we buy snacks from the tuck shop.On Sunday,we usually go to the bookshop.We often study together.
My best friend is really great!
2.my best friend
Perhaps you’d ask me,“Why do you like basketball so much?”Then I’ll say,“Because it is my best friend.It can make me feel happy when I am sad and I can also learn lots of things from basketball games.”
A few days ago,I bickered with my mother.Both of us were very angry.Then I left the room with my “friend”.
We went to a playground and played together for two hours.I didn’t know why I was happy.At last,I asknowledged my fault and apologized to my mother.She was happy,too!
I like my friends,so I also like basketball match.I usually watch it on weekends.My favorit basketball player is Tracy McGrady .He is very excellent.He once got thirteen points in thirty-five seconds.I think it’s a miracle.But he isn’t in good health,he often gets injured and I often worry about his health.
Last year,I represented my school to join ChenGuang Cup Basketball Match.Unfortunately,we was defeated.But we were still happy,because we did our best.At that time,I made a big decision,I’ll be a famous basketball player when I grow up.
Now,I’m practicing hard with my “friend”.So maybe I can join in NBA in many years.
I like basketball,I like my “friend”!
3.my best friend
I have a lot of friends in Simon English School.One of them is my best friend.His name is boyd.
He and I are both thirteen years old.but I am older than he,and I am fatter than he too.He is shorter than I,and he is thinner than I.He wears a pair of glasses but I don't.
He likes to eat pocket-food,and he is a gentleman in a sense that when he has pock-food he will share them with us.
He and I are in the same school and in the same class.So,we can help each other.We alway play in the school together.he likes to do thing I like.He lives in the Ningbo Jiangdong,so he hardly came to my home in Zhenghai but I alway go to his home on weekends and he sometimes will come to my home on weekends.Sometimes we will fight but we alway get along well.
His mother is a doctor in the Li-Hui-Li hospital and I know where her office is.His father is a worker in the Bei-Lun power plant.He is a head of a group.So,his father has a lot of work to do.
He and I are in the same school.We usually have a good time so he is my best friend.

因为friend前面已经有了限定词my的修饰.
而冠词和形容词性物主代词同属限定词,一般只能用其中一个.
再如：“我的老师/妈妈/名字”只需说my teacher/mother/name即可,楼主会说成my a teacher/mother/name或a my teacher/mother/name吗?
不过,可以说a good friend,因为good是形容词,并不属于限定词之列.
限定词一般指冠词/指示代词/形容词性物主代词/名词所有格.