Question

birds fly in the sky ( negative)
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Answer 1

Birds don't fly in the sky.

The sentence becomes negative after adding the word 'don't' before the verb, 'fly'.

By adding 'don't', the negative sentence becomes a contradiction of the sentence you have converted from (which is also known as an affirmative sentence).

This means that by adding the word 'don't', the negative sentence becomes opposed and contradicts the affirmative sentence in meaning.