There are three articles in English: a, an, and the. These are classified as indefinite (a and an) or definite (the).
Indefinite articles refer to something not specifically known to the person you are communicating with. In other words, a and an are used before nouns that introduce something or someone you have not mentioned before.
"I witnessed an eclipse this morning."
"I wrote a laboratory report before lunch."
A and an are also used when talking about your profession.
"I am an ethicist."
"I am a scientist."
Use a when the noun you are referring to starts with a consonant sound when pronounced.
"a city", "a factory", "a hotel", "a university"
If the word begins with a vowel sound when pronounced, then use an.
"an hour", "an umbrella", "an owl", "an igloo"
Use the when you know that the reader or listener knows or can identify what particular person or thing you are discussing.
"The results were confirmed."
"Did you unlock the door?"
You should also use the when the thing you are discussing has been mentioned previously.
"Each vector encoded a protein with a different reporter molecule. The size of the protein was..."
We also use the when talking about geographical features.
"the Tropic of Capricorn", "the English channel", "the Himalayas"
We also use the preceding certain nouns when it is known that there is only one of something.
"the sun", "the world", "the Imperial Palace", “the Pacific Ocean”