The article "the".

First of all, the word "the" is a lot more useful than people think. If someone says they are Bill Gates then I can ask them "Do you mean THE Bill Gates?" and then even if you don't know who Bill Gates is you know that Bill Gates is somebody famous. Or I can say "Go get the bottle of Coke in the refrigerator." and you know there is only one refrigerator and it has one bottle of Coke in it. Likewise, I can say "Go get the apple from on top of the table" and there should be one table with one apple on top of it.

"The" is used to refer to someone or something specific. For example, you might say "A boy lived at home. The boy got a call from a girl. The girl was his neighbor." Here we refer to "a boy" and "a girl" the first time they are mentioned and "the boy" and "the girl" the second time they are mentioned because the second time they are mentioned you know that they are the boy and girl we were just talking about in the previous sentence.

"The" is used between two people when both people know the person or thing being referred to. For example, if I tell you to "Get the Coke from the refrigerator" you know what refrigerator I mean and that there is a bottle of Coke in there. Similarly, you can tell me "Turn on the TV" and I don't have to ask which TV you mean. This is why we usually say "Go to the bank" or "Go to the post office" because we usually have a specific bank or post office in mind: otherwise, we could just say "Go to a bank" to "Go to a post office."

One exception to this is when we are talking about instruments: we can say "I can play the guitar" or "I can the piano" and we aren't talking about a specific guitar or piano. We could also say "I can play a guitar" or "I can play a piano". This is not necessarily wrong, although some native speakers might insist that you have to use "the" in these sentences. Note that leaving out an article altogether would definitely be wrong: you don't "play guitar" in the same manner that you "play soccer": it is not a game. Nor do you "play the computer" because it isn't an instrument.

Geographical terms with "the" do make sense though. We say "the United States", "the Great Lakes, "the Amazon River" and "the Atlantic Ocean" because we are talking about a specific group of states, a specific group of lakes, a specific river and a specific ocean. We do not say "the England", "the France" or "the Canada" however because these are proper names: the only reason why you would say "the Bill Gates" is if there were more than one Bill Gates and you wanted to specify that there was a specific Bill Gates who founded Microsoft.

Another use of "the" is in science when instead of talking about a specific member of a group you are talking about the entire group. For example, you can say "The Australian crocodile can grow to be ten meters long" and this would be the same as if you said "Crocodiles in Australia can grow to be ten meters long": the meaning is exactly the same except the former sound more pretentious than the latter. Oh and "pretentious" means using the words "former" and "latter" when you could have said "first" and "last".

Most native speakers of English don't realize that they change the pronunciation of the in front of most vowels. For example, in "the end", "the apple","the ice cream" and "the orange juice", the word "the" is pronounced with a long e sound. This is because of something French grammarians call "liason" and it means that the vowel sound at the end of the blends with the vowel sound that follows it. For the same reason, the "the" in "the umbrella" does NOT have a long e sound: saying "the umbrella" with a long e sound sounds very pretentious and we don't use a long e sound for that particular "the" because (listen as you say it) the sound at the end of "the" and the sound at the beginning of "umbrella" is actually the same sound.

And native speakers and non-native speakers alike often make the mistake of thinking that the article "an" comes before every word that starts with a vowel when in reality it come before every word that begins with a vowel sound: for example, we say "a uniform" and "a university" and not "an uniform" or "an university" because "uniform" and "university" start with the /ju/ sound and not the /u/ sound as in "moon". On the other hand, you do say "an hour" or "an MRT" because the h in hour is silent and MRT is spelled out and not pronounced as a word.

That reminds me: it is actually wrong to say "I like the NBA and the MLB". It's okay to say "I like the NBA" because that means "I like the National Basketball Association" and you are talking about a specific association but when you say "I like the MLB" you are saying "I like the Major League Baseball" and it sounds as if you are talking about a specific baseball. You should say "I like watching NBA and MLB games". Adding the word "games" also helps remind people you are talking about sports if they don't immediately know what the acronyms NBA or MLB mean.