1. Half of the bananas are from Mexico.

2. Half of my time was spent in reading essays.
3. Half of these boxes have stones.
4. Half of the money doesn't belong to me.
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Would you check the sentences? Are they all grammatical? Thank you for your help.

Yes, all are correct.

Yes, all the sentences you provided are grammatically correct.

To determine if a sentence is grammatically correct, you can follow these steps:
1. Identify the subject and the verb in the sentence.
2. Check if the subject and verb agree in terms of number and tense.
3. Look for any additional elements such as adjectives, adverbs, or prepositional phrases that modify the subject or verb and ensure they are placed correctly.
4. Check for correct word order and punctuation.

Now, let's analyze each sentence:

1. Half of the bananas are from Mexico.
- Subject: "Half of the bananas"
- Verb: "are"
- The subject and verb agree in terms of number and tense. The sentence is grammatically correct.

2. Half of my time was spent in reading essays.
- Subject: "Half of my time"
- Verb: "was spent"
- The subject and verb agree in terms of number and tense. The sentence is grammatically correct.

3. Half of these boxes have stones.
- Subject: "Half of these boxes"
- Verb: "have"
- The subject and verb agree in terms of number and tense. The sentence is grammatically correct.

4. Half of the money doesn't belong to me.
- Subject: "Half of the money"
- Verb: "doesn't belong"
- The subject and verb agree in terms of number and tense. The sentence is grammatically correct.

By analyzing the subject-verb agreement and verifying the sentence structure, you can determine if a sentence is grammatically correct or not.