English grammar worksheet focusing on the use of articles "a," "an," "the," and no article, designed for practice and learning.

To solve the problem, we need to determine whether to use a, an, the, or no article in each sentence. Here's a breakdown of the rules for articles:

1. "a" or "an": Used before singular count nouns when referring to something for the first time or in a general sense. Use "a" before consonant sounds and "an" before vowel sounds.
2. "the": Used before singular or plural nouns when referring to something specific or already mentioned.
3. No article: Used with plural count nouns, mass nouns (uncountable nouns), or abstract concepts when they are general.

Now, let's complete each sentence:

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

- The pen is specific because it is the one borrowed from the speaker.
- Answer: the

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

- The shirt is specific because it is the one mentioned in the previous sentence.
- Answer: the

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

- A new desk is being referred to for the first time, so use "a."
- Answer: a

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

- An old bike is being referred to for the first time, so use "an."
- Answer: an

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

- Water is a mass noun (uncountable) and is used in a general sense.
- Answer: no article

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

- The exercise book is specific because it is the one on her desk.
- Answer: the

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

- An old magazine is being referred to for the first time, so use "an."
- Answer: an

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

- Little bears are plural and specific because they are in the zoo.
- Answer: the

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

- "Hour" is a singular count noun, and "half an hour" is a common phrase.
- Answer: an

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

- The book is specific because it is the one just mentioned.
- Answer: the

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

- The man is specific because he is at the corner.
- Answer: the

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

- An ugly shirt is being referred to for the first time, so use "an."
- Answer: an

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

- A driver is being referred to for the first time, so use "a."
- Answer: a

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

- Friendship is an abstract concept and is used in a general sense.
- Answer: no article

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

- A pretty girl is being referred to for the first time, so use "a."
- Answer: a

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Final Answers:

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

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Boxed Final Answer:

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