I am really the right person to ask this question. I have two degrees in English, and one has an emphasis in grammar and linguistics. The first time someone asked me this kind of question, an international student asked me when I was a student, and I couldn’t answer, but I learned it is a very necessary question to answer for international students. I spent eight years teaching at a university in Romania, and I really had to think the grammar out and answer lots of questions like this, and I spent ten years teaching at Korean university and have had to answer even harder questions than this. Now a days, this question is like “A,B,C’s” for me. The countries that need “the” before them are “the United States of America,” “the United Kingdom,” “the United Arab Emirates of Saudi Arabia,” and another country that no longer exists: “the United Soviet Socialist States of Russia.” I will explain now why those need that article, “the.”
To begin with, most names don’t need an article. For example, if I just said “America,” “Russia,” or “Saudi Arabia,” none of those need “the.” They need to begin with a capital letter because they are unique and special. There is only one Russia, only one America, and only one Saudi Arabia. If you use your name, you must also capitalize your name because there is only one of you. For example, the name of President Moon of South Korea must be capitalized. The name of America’s president “Donald Trump” must be capitalized. If you talk about Ceausescu or Illiescu from Romania or Putin from Russia, their names must be capitalized. There is only unique entity called Donald Trump. The names are called proper nouns, and proper nouns must be capitalized, and never take an article.
However, “the United Kingdom,” “the United States of America,” “the United Arab Emirates of Saudi Arabia,” and “the United Soviet Socialist Republic of Russia” are phrases, not just one word. Grammar rules apply. A noun is a person, place, or thing. The name of a country or the name of a person is a noun, a proper noun. If you use a word like “table,” “computer,” or “chair,” they are count nouns. You can count them, and there is more than one table, computer, or chair in the world. They are things. In the names of these countries, there are things too: “state” is a thing; “emirate” is a thing; “kingdom” is a thing. These words are count nouns. Non count nouns also exist. Proper nouns are a form of non count nouns, but other count nouns are words like “water,” “air,” “pudding,” or “milk.” You can’t count those things, so you can’t put an “s” on the end and you can’t put “a” in front of them. You can put “the” in front of them if they are special. For example, you can say, “the water in my glass.” That is special water. It is not just any water. If you say, “the air in this room,” that is special air. “The” makes something special, and you can use it with count or non count nouns. “A” means “one,” and you can’t put “a” before a word like “water” or “pudding.” You can say, ” a glass of water,” but in that case, “a” is matched with “glass,” not with “water” because the only way you can count the word water is by putting it in a glass or in a bottle. You can say “a dish of pudding” or “an ice cream cone,” but not “an ice cream” or “a pudding.” If I say, “Let’s go get a Coke,” the idea is a bottle of Coke or a glass of Coke. The word “Coke” really can’t be counted because it is liquid. You need to think “glass” or “bottle” to count it. With ice cream, you need to think “cone” because “cone” is the solid thing that can be counted, and ice cream is frozen liquid that can’t be counted.
When you say “the United Kingdom,” you are not talking about just any kingdom. If you were talking about any kingdom, you would say “a kingdom” because the word “kingdom” can be counted, so it needs either “a” or “the” before it or “s” after it. ” A” before a word means “one,” and if the noun begins with a vowel or a vowel sound, you must use “an,” for example “an ice cube” or “an apple.” “I’ is a vowel. “A” is also a vowel, so these words need “an” instead of “a” before them. When you put an adjective before your noun, you qualify the noun. “United” describes or qualifies the noun “kingdom,” so it is an adjective, and “the” must be placed before the adjective. If you use “the” before a noun, it makes that thing special and unique. It means there is only one. “A kingdom” means any kingdom. “The kingdom” means a special kingdom. “The United Kingdom” means a special kingdom, and it describes the kingdom even better than just saying “the kingdom.”
If you say “the United States of America,” “States” is your count noun. When you talked about “the United Kingdom,” there was only one kingdom, but when you talk about “the United States of America,” there is more than one state, so you must use the “s” on the end of “state.” Using “united” before “states” qualifies or describes “states,” so “united” is an adjective again. Since these are special states, you need to use “the.” “Of America” is a prepositional phrase. It begins with a preposition, “of,” and ends with the proper noun that becomes the object of the preposition “America.” The prepositional phrase qualifies or describes “states” even more.
Again, we have “the United Arab Emirates of Saudi Arabia.” In this phrase, “emirates” is the count noun, and there is more than one emirate, so “emirate” needs an “s” on the end. These are special, unique emirates, so you need “the.” Again “united” describes “emirates” like it described “states” and “kingdom.” “Arab” also describes “emirates” in this case. “Arab” is an adjective. “Of Saudi Arabia” is the prepositional phrase. “Of” is the preposition. “Arabia” is the proper noun, and it is the object of the preposition. “Saudi” is an adjective that tells you about “Arabia.” The whole string of words needs “the” in front of it because “Emirates” is a special count noun.
If you didn’t understand the grammar before, I hope you are beginning to understand. “The United Soviet Socialist Republic of Russia” has the same kind of grammar. “Republic” is the special count noun that needs “the” before it. “United Soviet Socialist” is a string of adjectives that describe “Republic.” Again “of Russia” is a prepositional phrase. “of” is the preposition, and “Russia” is a proper noun that is used as the object of the preposition.
This seems very technical, but it is the kind of questions I have had to learn to answer. Just the average English speaker never has to think about this grammar. It is innately learned as a child, and we don’t need to think about it. However, when people study English as a second language, they think about this kind of thing. If they don’t, they could get it wrong, and they want to understand why that native speaker is looking at them funny or why the teacher has marked their papers wrong. I have answered so many questions like this, and my students learned how to do it right. They had to understand the grammar to use it right. It had to make sense to them so they could apply the grammar correctly every time. Americans and British just use it without thinking. We know what sounds right, and we use it, but not many of us can explain it. It takes an English teacher who understands grammar well to figure these kinds of questions out. In some countries, they think they can hire anyone with a degree, even if it isn’t English, to teach English, and they can give you some, but they can’t explain these kinds of questions. I have actually had other American or British English teachers who are teaching in foreign countries call me up and ask me this kind of questions because their students asked them, and they couldn’t explain it. That comes because they hire people to teach English who have not had to study grammar.