After more than ten years of learning English, I still have a few problems with articles.
Which of the following two sentences is correct and why?

A cow is an animal.


The cow is an animal.


Both can be considered correct, depending on the situation. "A" and "an" are considered indefinite articles. Therefore, when you say "a cow" or "an apple", you wouldn't have a definite/particular cow or apple in mind. "The" is a definite article. Therefore, when you say "the cow", you would have a definite/particular cow in mind.

In some sense, though, my answer is over-generalized. The sentence "the cow is an animal" could also refer more generally to the taxonomical genus to which cows belong. However, even this would still be a definite use of "the" because you have a particular genus in mind.


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