Writting

I am studing English. Can you indicate my errors? Thank you for your comments.

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Dear Kathy,
Many thanks for your recent letter. I'm sorry I haven't invited you to my mother's birthday party on the last Monday. I knew that you wasn't in the town last week because of the tour of Phu Quoc Island.
As you know, my mom has just 50 year old this year. Therefore, my father, my sister and I decided to hold a surprise birthday party as thanks to her love for my family. The party was organized at our home. In the afternoon, after my mother had finished her work at the office, my father pick up her by the car. While, my sister prepared dinner and I bought a birthday cake at a local store.
Because everyone was busy for work on Monday, we only invited my uncle, my ant and Tom, who is my sister's boy friend. Although it only was a small party, my mother was very happy.
Now, I guess in's time I got on with my study. I look forward to hearing about your trip soon.
Best wishes,
Tan