3 An apple a day keeps the doctor away. In this sentence, we use 'the doctor' to mean a known doctor: perhaps the only doctor in a village or neighbourhood, or the doctor who always treats your family. It's a specific use of 'the', meaning 'local' or 'habitual'. We use 'the' in this way to refer to one's usual doctor, dentist, barber and so on, and also in phrases such as 'go to the gym', 'go to the store' or 'go the pub'. We wouldn't use 'a' here, because this would make us wonder 'Why only one doctor?'. You could use the plural 'doctors'. This would be fine grammatically, but a little odd (like keeping flies or mosquitos away?). As I'm sure you know, this is a saying meaning 'keeps you healthy', and is a traditional fixed phrase. Once again, this is an unusual example of article use. It is therefore not a good example to put in a text book that's meant to be teaching the basics of article use.

When DO I NEED to use an article before a noun? The most important general rule to remember is that if a word is countable and singular, you need to put an article before it. [I say 'general rule', because there are exceptions, especially in adverbial phrases such as 'at home', 'in bed' and 'to work']. When deciding which article to use, remember that we use 'a' and 'an' to mean 'one of several'. For example, if there are several apples in a bowl, you might ask 'Can I have an apple?' This means any one of a number of apples, rather than a specific one. But if there is only one apple in a bowl, you'd ask 'Can I have the apple?', because we all know WHICH apple you are talking about (the only one in the bowl). I hope that's clearer now. Let's look at your sentences: 1 An apple is a hard round fruit with a green, red or yellow skin. The 'a' before 'skin' is optional. The noun 'skin' is usually uncountable, but in this case it you could use 'a' to emphasise the difference between ' a green skin', 'a red skin' and so on, as if these were different kinds of skin. Personally, I think this is a really bad example to put in a textbook for students who are struggling with the basic concepts of articles. The writer should have given the example "An apple is a hard round fruit with green, red or yellow skin", as this follows the rules better. It isn't wrong to use 'a' before 'skin', but it's unfair to put this unusual example in a textbook exercise about articles. 2 An apple is a round fruit with smooth green, yellow, or red skin and firm white flesh. This would be correct with or without 'a'. Without the article, we're using 'skin' as a uncountable noun. With the article, we're using 'a smooth, green, yellow or red skin' as a shorthand for 'a smooth, green, yellow or red [kind of] skin'. The 'a' refers to the idea of 'kind of skin'. In my view, this is a poorly written textbook. How can students learn about articles when the examples are so inconsistent?

Hello, Ivana Hoxha. Sentence 1 , I copied it in a English-Chinese dictionary used in my country, so I didn't doubt it. Thank you for your explanation. A great help for me.

Hello, Tim Clouter. Thank you very much.

A is an indefinite article, used for non specific items; it could refer to any apple at all, any apple is acceptable. An apple is red, and has a skin etc etc, but the apple is the specific apple we are talking about - the one I have in my hand, for example. "The" is a specific article - the apple you are specifically referring to, and the only apple that matters at the moment. It is what is known as a definite article. I want an apple - I want an apple, any will do. I want the apple - I want the specific apple we are talking about. You can't use "Keeps a doctor away" because although grammatically correct, it is not the expression that is used and therefore would sound odd to an English speaker! I hope this makes sense, if you've never used articles before it can be difficult!