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Indefinite Article (a/an)

Learn how to use the indefinite article *a/an*, and see some examples in a text about linear regression.

Indefinite Article


There are two indefinite articles: a and an. An is used before words that start with a vowel sound, a, e, i, o, u. For example: an organisation, an effect, an accurate analysis.

A is used before words that do not start with a vowel sound. For example: a graph, a relationship, a model, a database.

Be careful, sometimes a word starts with a vowel, but it is not said with a vowel sound. This is common with words that start with the letter u but which have the sound y. For example: university, unit. For these words you need to use a: a university, a unit.


The indefinite article, a or an, is used before singular nouns. It has a number of uses:

1. For the first time

A is used when you refer to something for the first time, so people do not know what it is.

Here’s an example:

It is sometimes called a trendline or a line of best fit.
In the example, this is the first time the text has said trendline and line of best fit, so they use a not the. If they referred to them again later in the text, they would write the trendline and the line of best fit.
2. For a non-specific noun
Use a or an to refer to a noun that is part of a group, but which you have not defined or identified. In other words, you are not referring to a specific example.
Here’s an example:
In a graph, the independent variable is the x-axis (the horizontal line) and the dependent variable is the y-axis (the vertical line). You can see these labelled in the graph below.
At the beginning of the sentence, a graph is used because it is not referring to a specific graph. At the end of the sentence the graph is used because it is referring to the specific graph shown below the text.
3. To mean one
Use a or an to mean one of something.
Here’s an example:
Linear regression is a model used in data science.

We use a here because there is more than one model in data science. If linear regression was the ONLY model, you could say linear regression is the model used in data science.

Task 1

Here are some more examples from the reading passage. Can you work out why the indefinite article, a or an, has been used in each example?

Example Reason used
For example, linear regression might be used to answer a question like this …  
Therefore, in the example, productivity changes as a result of how many hours of sleep a person gets.  
In a linear relationship, there is also “negative correlation” and “no correlation” as you can see in the graphs below.  

You can download the answers here: Task 1 ANSWERS

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English for STEM: Understanding Technology Vocabulary

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