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.

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

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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.

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

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

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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. 

The word "every" is commonly used with time-related phrases to indicate regular intervals or repeated actions. In these constructions, "every" helps to establish a schedule or frequency, making it clear how often something happens within a given time frame.

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First of all, "every" can be used with any unit of time, from a second to a century, to indicate that something happens once during that time period. "Every day" means that something happens once in 24 hours, whereas something that happens "every year" happens at least one time during a 365-day period.

Otters have a high metabolism, so they have to eat almost a fifth of their weight every day.

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In February, every year, we host a large pond hockey tournament.

Caption 10, Adventures with Kate: Pond Hockey

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We also use "every other" to talk about a predictable frequency that skips time. For example, "every other day" means the event happens one day, then skips the next day, and happens again the day after that. 

I have to go shopping almost every other day.

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Always living out of a bag... being in a different location every other week.

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Sometimes, we also use "every" with "few" or "a couple" and a plural noun. Have a look: 

I just get touched-up every... every few seconds.

Caption 74, Food Diaries: Everything Kylie Jenner Eats in a Day

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It could be every couple hours or every half hour.

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And sometimes, the interval has a number that really needs to be specified. These sentences state that something happens once in a four-day period and once in a six-week period. 

The eaglet can gain more than a pound every four days.

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The windows have to be cleaned once every six weeks.

Caption 14, In London: with Lauren Buckingham Palace

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Further Learning
You will find many examples likes these on Yabla English. For an overview of the difference between "each" and "every," see our related lesson.