In English, there is also the word "an" that is used instead of "a". "a" or "an" is used to refer to non-specifc noun. "the" is used to refer to specific noun. Examples: 1. I want to buy a house. 2. I want to buy the house I'm renting. In example 1, I am saying in general that i want to buy a house, any house, without any specific house in mind. In example 2, I am saying that I want to buy that exact same house that I am renting. You will use "an" if the noun starts with a vowel. Example: 3. I want to buy an umbrella. More examples: 4. My country has a president. 5. The president of my country is John. 6. I need to see a doctor. 7. The doctor told me to exercise regularly.