English grammar exercise focusing on the correct use of articles "a," "an," "the," and no article in sentences.

To solve the problem, we need to determine the correct article (a, an, the, or no article) for each sentence. Here's a detailed explanation and solution for each sentence:

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1. "Where is ________ pen you borrowed from me yesterday?"

- Explanation: The pen being referred to is specific because it is the one borrowed from the speaker. Use the.
- Answer: "Where is the pen you borrowed from me yesterday?"

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2. There is a blue shirt on sale. Unfortunately, ________ shirt is not my size.

- Explanation: The second mention of "shirt" refers back to the specific blue shirt mentioned earlier. Use the.
- Answer: "There is a blue shirt on sale. Unfortunately, the shirt is not my size."

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3. "I need ________ new desk for my work."

- Explanation: A new desk is being requested, and it is not a specific desk yet. Use a.
- Answer: "I need a new desk for my work."

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4. Mike has ________ old bike. His bike doesn't ride well.

- Explanation: The bike is described as "old," which makes it specific to Mike. Use an (since "old" starts with a vowel sound).
- Answer: "Mike has an old bike. His bike doesn't ride well."

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5. ________ water is important for life on the Earth. We must keep it clean.

- Explanation: Water is a general concept here, so use no article.
- Answer: "Water is important for life on the Earth. We must keep it clean."

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6. Lisa has ________ exercise book on her desk. The exercise book is blue.

- Explanation: The first mention of the exercise book is specific because it is on her desk. Use the.
- Answer: "Lisa has the exercise book on her desk. The exercise book is blue."

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7. I am reading ________ old magazine now. It's very interesting for me.

- Explanation: The magazine is being read for the first time in this context, so it is not specific. Use an (since "old" starts with a vowel sound).
- Answer: "I am reading an old magazine now. It's very interesting for me."

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8. We saw ________ little bears in the zoo. The bears were funny.

- Explanation: The bears are specific because they are in the zoo. Use the.
- Answer: "We saw the little bears in the zoo. The bears were funny."

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9. You have only half ________ hour for lunch. Hurry up!

- Explanation: "Hour" is a singular noun, and it is specific in this context. Use an (since "hour" starts with a vowel sound).
- Answer: "You have only half an hour for lunch. Hurry up!"

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10. I bought a book and put it in my bag. ________ book is very heavy.

- Explanation: The book is specific because it was just bought and put in the bag. Use the.
- Answer: "I bought a book and put it in my bag. The book is very heavy."

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11. I am sure ________ man at the corner is waiting for you. He is looking at us.

- Explanation: The man is specific because he is at the corner. Use the.
- Answer: "I am sure the man at the corner is waiting for you. He is looking at us."

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12. He was wearing ________ ugly shirt when I first met him.

- Explanation: The shirt is being described for the first time, so it is not specific. Use an (since "ugly" starts with a vowel sound).
- Answer: "He was wearing an ugly shirt when I first met him."

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13. Robert is ________ driver. He drives big trucks.

- Explanation: The job title "driver" is being used generically here. Use a.
- Answer: "Robert is a driver. He drives big trucks."

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14. I believe that ________ friendship plays an important role in people's lives.

- Explanation: Friendship is a general concept here. Use no article.
- Answer: "I believe that friendship plays an important role in people's lives."

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15. Laura is ________ pretty girl. She looks very smart, too.

- Explanation: Laura is being described as a pretty girl for the first time. Use a.
- Answer: "Laura is a pretty girl. She looks very smart, too."

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Final Answer:

1. the
2. the
3. a
4. an
5. no article
6. the
7. an
8. the
9. an
10. the
11. the
12. an
13. a
14. no article
15. a

Boxed Final Answer:
\boxed{1. \text{the}, 2. \text{the}, 3. \text{a}, 4. \text{an}, 5. \text{no article}, 6. \text{the}, 7. \text{an}, 8. \text{the}, 9. \text{an}, 10. \text{the}, 11. \text{the}, 12. \text{an}, 13. \text{a}, 14. \text{no article}, 15. \text{a}}