"The" article

[Planetypus]

I think both of the following sentences are correct:

Cow is a useful animal.
The cow is a useful animal.

If so, what is/are the difference(s) between the two sentences?

I guess the meaning changes when we use the. Like, the first sentence could mean that all cows, in general, are useful (including the sickly). But the second sentence could mean that that specific cow I, and the person spoken to, know is useful.

I want to hear your opinion. Thanks a lot!

[Mister Micawber]

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Only your second sentence is correct. A singular countable noun must be fronted by an article, a demonstrative, or a possessive adjective.

The cow refers to all cows (or you can simply use 'Cows') unless the context indicates a specific beast or mentions it previously.
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Native English teacher at Mister Micawber's

[Tom]

Sorry, but I am terribly baffled!

[Mister Micawber]

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I have not read that thread, Tom, but I can only think that Alan was considering 'apple' as a substance or a word per se. In the latter case, 'apple' would better appear in quotes or italics:

'Apple' is a kind of fruit.

My opinion remains unbowed: singular countable nouns must be modified by an article, a demonstrative, or a possessive:

The apple is a noble fruit.
This apple is a noble fruit.
John's apple is a nobel fruit.
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Native English teacher at Mister Micawber's

[Planetypus]

Mister Micawber,

Thanks a lot. Your response makes a lot of sense to me.

(How do you pronounce your Micawber?)

[Tom]

[Planetypus]

thanks