1. He is busy during the weekend.

2. He is busy during the weekends.
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What is the difference between them?

The first weekend is singular; the second is plural.

The difference between the two sentences is the use of the singular form "weekend" in the first sentence and the plural form "weekends" in the second sentence.

1. "He is busy during the weekend."
This sentence indicates that the person is busy specifically during one particular weekend, without specifying which weekend it is. It suggests that this is a recurring pattern and that the person is consistently busy during any given weekend.

2. "He is busy during the weekends."
In contrast, this sentence suggests that the person is generally busy on weekends, without specifying any specific weekend. It implies that the person is typically occupied or has commitments that extend to most, if not all, weekends.

So, the main difference lies in the specificity of time. The first sentence refers to a specific weekend (though unknown), while the second sentence refers to weekends in general.