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Here are the mistakes and corrections for each sentence: 1. There's been an accident - dial 999 and ask for the ambulance. Correction: None needed, the sentence is grammatically correct. 2. I heard an child crying. Correction: I heard a child crying. (Added article 'a' before 'child') 3. He is a engineer at a large electronics company. Correction: He is an engineer at a large electronics company. (Changed 'a' to 'an' before 'engineer') 4. We've been having a few problems with a new computer. Correction: None needed, the sentence is grammatically correct. 5. Let's go to movies this evening. Correction: Let's go to the movies this evening. (Added article 'the' before 'movies') 6. I just bought a new shirt and some new shoes. The shirt was pretty expensive, but shoes weren't. Correction: I just bought a new shirt and some new shoes. The shirt was pretty expensive, but the shoes weren't. (Added article 'the' before 'shoes') 7. I really enjoyed a book I've just finished reading. Correction: I really enjoyed the book I've just finished reading. (Added article 'the' before 'book') 8. You're the fifth person to ask me a question. Correction: None needed, the sentence is grammatically correct. 9. She lives in a special home for a poor. Correction: She lives in a special home for the poor. (Changed 'a' to 'the' before 'poor') 10. A panda is becoming an increasingly rare animal. Correction: The panda is becoming an increasingly rare animal. (Changed 'a' to 'the' before 'panda') 11. I like travelling by the train. Correction: I like travelling by train. (Removed article 'the' before 'train') 12. It's very hot here in the summer. Correction: It's very hot here in summer. (Removed article 'the' before 'summer') 13. They were having the breakfast when I arrived. Correction: They were having breakfast when I arrived. (Removed article 'the' before 'breakfast') 14. There is the beautiful garden behind his house. Correction: There is a beautiful garden behind his house. (Changed 'the' to 'a' before 'beautiful') 15. She studied the chemistry and physics at college. Correction: She studied chemistry and physics at college. (Removed article 'the' before 'chemistry' and 'physics')