Today's tip: a和the的用法

一些词：
specimen (n.) 样本，样品
jigsaw puzzle (n.) 拼图

Welcome to Daily Tips on learning English. Today’s tip is on using the articles “a” and “the” correctly.
More specifically, today’s tip is on the difference between using “a” or “an” before a singular countable noun and using “the” or “the(i:)” before a singular countable noun. Most frequently, both “a” and “the” are used to refer to a specimen of a group of things, for example, “a table” and “the table” both refer to one table out of the group of all tables. When people say “the table”, they mean the table that we’ve been talking about and which you know about. So if you begin to talk about a new object about which your listener knows nothing, you must use the article “a”. But after you have introduced the object, or future references are preceded by the article “the”. Let’s take a simple example.“I bought a table yesterday. The table is in the living room. I really like the table.”Did you notice how the article “a” was used first, and then after that, the article “the” was used. If you said, “I bought the table yesterday” instead of “I bought a table yesterday”, you would mean that you and the listener are talking about some table previously. If you hadn’t talked about any table previously, the listener would be confused and would probably ask you, “What table?”
Let take another example.“I put together a jigsaw puzzle yesterday. Really? How often do you put together a jigsaw puzzle?”Notice that the article “a” was used in both sentences before “jigsaw puzzle”. That’s because the question, “How often do you put together a jigsaw puzzle” was not referring to the same jigsaw puzzle mentioned in the first sentence. If you ask, “How often do you put together the jigsaw puzzle?” you would be asking how often the listener put together the same jigsaw puzzle, you would be saying the person puts together, then takes apart, then puts together the same jigsaw puzzle over and over again. And that’s a silly thing to ask.
So remember, “the” is used to refer to a previously mentioned specimen, something about which the listener knows. The article “a” is used to refer to a specimen which hasn’t been mentioned before, something which the listener doesn’t know about.
This has been today’s tip on learning English. Tune in tomorrow for another tip.