The indefinite articles "a" and "an" are used for singular countable nouns—like "a tree" or "an apple." While we say "the sun" because there is only one, we use indefinite articles when talking about something that is neither specific nor unique, and particularly when we mention it for the first time. 

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Look, if you want, we can go and get a tree this weekend.

Caption 38, Falling For Christmas: Full Movie

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And I pre-slice an apple by my bed.

Caption 102, Food Diaries: Everything Kylie Jenner Eats in a Day

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The simple explanation is that "a" is used for nouns that begin with a consonant and "an" is for those that begin with a vowel. But this actually relates to the sound the word begins with, not the word's spelling! In the words "hour" and "honor," the "h" is silent, whereas "university" begins with a "y" sound. This is what determines the appropriate article. 

You could just drop by the shop in about an hour.

Caption 54, Falling For Christmas: Full Movie

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Well, it's such an honor to be here. I'm so honored.

Caption 14, The White House: Jimmy Fallon at the White House

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Technically, a university is generally a school that offers an undergraduate and a graduate program.

Captions 38-39, Ask An American: COLLEGE, UNIVERSITY, and HIGH SCHOOL in the U.S.

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It's important to note that if there is an adjective before a noun, this is what determines whether "a" or "an" is used. You can see below that even though "idea," "ability," and "honor" all start with a vowel sound, the preceding adjective begins with a consonant sound, which requires "a." The opposite is the case in "an interesting history."

Jim, if you ask me, and you haven't, I think this is a terrible idea.

Caption 34, DeForest Kelley: Dr McCoy Star Trek

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English has shown a unique ability to absorb, evolve, invade, and, if we're honest, steal. 

Captions 4-5, The History of English Global English

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Uh, so, Mr. President, it was a great honor being with you.

Caption 44, Global News: Barack Obama and Donald Trump meet at White House

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This is Tranquille River, and it has an interesting history.

Captions 38-39, Merve: Outdoor in Canada

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Further Learning
You'll find many, many examples of "a" and "an" used on Yabla English. 

Many people travel during the holiday season, and although train travel is common in the UK and air travel often necessary in the US, a lot of people drive to see their loved ones. Let's look at vocabulary related to cars and driving. 

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Unless you are driving an electric car, you will have to "fuel up" at the beginning or "stop for gas" along the way. 

Let's find a gas station, and I'll put some in the engine.

Captions 13-14, Basic Vocabulary Common: Words in Context

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Now, with the price of fuel and the distances we have to travel to get it...

Caption 5, Once upon a Time... Planet Earth: The Climate in the Far North

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Making sure that your car has been inspected recently and undergoes regular maintenance is essential for safety reasons, as is abiding by the speed limit and using a seat belt. 

"Oh no, I forgot to change the oil in my car!"

Caption 11, Basic Vocabulary: Common Words in Context

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It's so easy to go over the speed limit without realizing.

Caption 51, Get to know: The Town of Speed

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And be sure to fasten your seat belt.

Caption 61, Air New Zealand An Unexpected Briefing

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Although driving eliminates the need to book in advance or leave at an exact time, it does have a few disadvantages. One of these is dealing with traffic. 

But they can be a bit slow in the morning or evening traffic.

Caption 13, London: London Transport (A1-A2)

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So a traffic jam happens when the road is blocked.

Caption 37, Sigrid explains: Traffic

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While a road trip implies a car journey with multiple stops, a commute is a regular trip to and from the same place. When people carpool, they share a vehicle in order to decrease their environmental footprint. 

"Do you want to go on a road trip with me?"

Caption 4, Cabrinha Kiteboarding With Susi Mai in Ireland

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It became practical for people to live on Long Island and commute to the city for work.

Captions 8-9, Traveling with Sigrid Exploring Long Island, NY and Princeton, NJ - Part 9

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We had to carpool so he rode with his legs hanging out the window.

Caption 5, The White House: Jimmy Fallon at the White House

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Further Learning
You can watch Sigrid's video on traffic, as well as other videos related to car travel, on Yabla English.