Объясните наличие или отсутствие артиклей перед существительными в следующих предложениях

1. This is a pencil. It's a red pencil. The red pencil is on the table. 2. I'm a teacher. I'm at home now. This is my room. The room is large and clean. 3. Open your books at page thirteen. 4. This is a black tie and that is a white tie. 5. Yesterday I met my friend with a young girl. The girl is a first-year student of the University. She lives on the second floor in the house not far from us. 6. Summer is the most beautiful season of the year. There are many flowers in the fields and gardens everywhere. The flowers are very beautiful in your garden. 7. Victory Square is in the centre of Minsk. 8. I live in Kirov Street. 9. The Black Sea is in the South of our country. 10. I live in the Republic of Belarus but my friend lives in Moldova.