1. We use ‘a’ / ‘an’ to introduce new information. The listener does not know which person or thing we are talking about.

I have a suggestion to make.
I’m not unemployed any more. I’ve started a business of my own.

2. We use ‘a’ / ‘an’ to refer to something for the first time.

I have an idea I’d like to discuss.
I’m going for an interview at Siemens next week.

3. We use ‘a’ / ‘an’ to refer to one of a group of things.

Can you pass me a paper clip from that box?

4. We use ‘a’ / ‘an’ to describe someone’s job.

Fiona is a financial consultant.

5. We use ‘a’ / ‘an’ with an adjective to describe something.

Cairo is an enormous city.

6. We use ‘a’ / ‘an’ in expressions of measurement. ‘Per’ can also be used.

Twice a/per month
300 units an/per hour
We charge €200 an/per hour.

7. We use ‘a’ or ‘an’ to mean ‘one’.

I get over a hundred emails every day.

8. ‘a’ / ‘an’ mean ‘one’, so we cannot use ‘a’ / ‘an’ with plurals or uncountable nouns. We use ‘the’ or ‘some’.

Can you give me the/some information?
This report is about the/some research we’re doing?

9. We use ‘a’ or ‘an’ when the person or thing we are talking about is not specific.

We need to call a technician.