The correct use of articles, 鈥榓鈥, 鈥榓n鈥 or 鈥榯he鈥, is one of the most difficult things to learn in the English language. Few native English speakers would be able to explain why they chose to put 鈥榓鈥, 鈥榓n鈥 or 鈥榯he鈥 in front of some words but not others, and there are even variations among those who do know.
If you are a total beginner a safe rule would be - if in doubt put them in - you are more likely to be right than wrong!
Here are a few of the more basic rules - for a more comprehensive overview see Swan聽Practical English Usage.
- Articles are NOT normally used with uncountable nouns or plurals. E.g.聽I feel full of energy. Holidays are fun.聽They are almost always used with singular countable nouns. E.g.聽础听book is聽补听good way to pass聽迟丑别听time on聽补听rainy day.
- Articles are used when we are writing about particular things, but are not usually used for things in general. E.g.聽罢丑别听water was cold.聽We are talking about a particular swim or drink perhaps.聽Water is essential for living things.聽Now we are talking about water in general.
- 鈥楾he鈥 is a definite article. It is used when we expect someone to know who or what we are talking about. E.g. "The boy over the road has a skateboard." We know who the boy is, and which road we are talking about, but there is nothing special about the skateboard. If we said "The boy over the road has聽迟丑别听skateboard." the meaning has now changed. Now we also know something significant about the skateboard - maybe it is one that was lost, or it is one that the listener also knows about.
- 鈥楢鈥 or 鈥榓n鈥 can only be used with singular nouns. 鈥楢n鈥 is used in front of words which start with a vowel - 鈥榓, e, i, o, u鈥. E.g.聽An apple fell off a tree.聽鈥楢n鈥 is also used in front of acronyms or words which聽蝉辞耻苍诲听as if they start with a vowel. e.g.聽An FSA is a finite state automaton.
There are many more details and exceptions to the use of articles that the more advanced speaker should get to know - Swan has approximately 14 pages on the use of articles - it is an excellent reference book for NESB students.