Articles are a type of adjective.

Russian speakers, in particular, have trouble knowing when to use articles. Actually, pretty much everyone has trouble with them, apart from Arabic and western Europeans and their descendants.

Dmytro asked: "Why do we say 'I like the sea" but "I will travel by sea'?" That is because we use "the" with things that are unique: thus, the sea, the sky, the sun, the rain.

Why "I have a car" but "I travel by car". In this case, the car is not unique, and it is a countable noun. Since it is not a specific car which the hearer has heard before, we use the indefinite article to describe it.

See this article by Grammar Girl.

In one of Margarita's recent letters, she wrote she wanted to complain about "burning rubbish at my neighbors house". I am going to suggest to her that is would be better to complain about "the burning of rubbish". This illustrates one way articles can be used to differentiate meaning. "Burning rubbish" sounds like the rubbish has caught fire by itself, while "the burning of rubbish" sounds like someone is setting it alight.

As I pointed out, we don’t use “the” before most place names, like Seoul, or Sydney, or Australia. There are some exceptions, like the United States of America, but in this case, the “United States of” is a prepositional phrase. Therefore, you could say, the city of Sydney, the Commonwealth of Australia, and so on.