Why do people say "How to use an iron" meaning any iron? Doesn't it mean that there is one iron and you ask how to use it and that because we can't say "I love a cat" because it means one cat? Shouldn't it be "How to use the iron" meaning not a particular iron but any iron in the world or rephrased with the plural form. And why in the "A X is a Y" or "A X is…" (for example: A dog is an animal) sentences the indefinite article is used as "any" but not as one of the class of X. I can't say "Cigarettes are bad for a body" (with meaning every/any body) but I can do that in the first example with an iron generalizing the word.

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Normally you use "a" or "an" to mean one when you are not referring to a specific one, and you use "the" when you are referring to a specific one. So "I will show you how to use an iron" is general - any iron would do. "I will show you how to use the iron" would usually refer to a specific iron.

There are exceptions. In "I will teach you to play the piano" we use "the" even though it applies to any piano.

Plural forms can be confusing. "I can play castanets", because I use two together. If you say "I can use irons" you invite a reader to ask why you used the plural. Do you use more than one? Are some different from others, needing different skills?

Some of your other examples are correct but might not mean what you want. Context is important and the same construction can have different meanings. "I love a cat" means there is a cat somewhere that you love. "I love the cat" means you love that specific one. If you used "own" instead of "love" the constructions work the same way. But while "I love cats" means you love all of them, "I own cats" only means you own more than one.

"Cigarettes are bad for a body" is not used because using the indefinite article suggests there are more than one. Using "the body" refers to the only one you have. "Anybody" and "any body" are not the same.

You have pointed out some interesting variations in how the articles are used. I think the best strategy is to hear and read as many as possible - the rules are not always clear.

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    The usage "I love a cat" to mean "I'm fond of cats" existed in the past, but would sound odd in modern usage, except possibly with an adjective - "I love a good apple pie." The novelist Patrick O'Brian, who wrote pretty convincing early 19th century dialogue, has characters say things like "I love a wench, it is true" (meaning that he's highly sexed, not referring to one particular girl), and "I love an oboe" (today we would say "I love the oboe"). Apr 17, 2021 at 9:41
  • But why does "a" with iron mean any iron but "a" with cat mean one cat? Does "to use" make it clear to understand that "a" means any?
    – zhabometr
    Apr 17, 2021 at 9:42
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    In both cases there is just one, but I am not saying which. Compare "I used an iron", which is only about one. If I love a cat it is only a specific cat, but if I can use an iron I can use not only the one I have used, but I have shown the skills to use any similar iron.
    – Peter
    Apr 17, 2021 at 10:49
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    I agree that the rules of article usage are complex and not always obvious. But I think "How to use an iron" and "I love a cat" are not that different. In both cases, there is only one object at a time that is being used or loved, and we are not being specific about which one it is. "an iron" = one unspecified member of "all irons"; "a cat" = one unspecified member of "all cats".
    – stangdon
    Apr 17, 2021 at 13:47

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