§ Which of these ("a" or "an") should you pick? That depends on the sound that begins the word immediately following the article.
§ If that word starts with a consonant sound, use "a". For example, it's correct to write "a boy", "a paper", "a sunny day" or "a European" (because the word "European" begins with the consonant "y" sound).
§ If that following word starts with a vowel sound, use "an". You should say "an experiment", "an envelope", or "an empty glass".
But what if you're referring, in a general sense, to more than one member of a group?
§ Now you need the word "some". For example, it's right to talk about "some books", "some letters", or "some people".
Does the noun you're using refer to a specific member of a group?
§ If you're referring to a specific member of a group, use the definite article "the". For example, you're correct to write "the experiment proved that ...", or "the method we used ...".
§ Even if you're referring to more than one specific member of a group, you should still use "the". It's right to say, for example, "the excellent papers written by Dr. Chen" or "the exams I took last week".
Does the noun you're using refer to something unique?
§ If it does, use "the". For example, you can talk about "the Louvre", "the planet Venus", or "the theory of relativity". There is only one -- a very special one -- of each of these.
Does the noun you're using refer to something that cannot be counted?
§ If so, you must use "the". You must say, for example, "I spilled the milk", or "I fell into the water". We can count glasses of milk or bottles of water, but not milk or water themselves.
Does the noun you're using really need an article?
§ If you want to express a general meaning referring to a group of very similar things, you should omit the article. Thus, you can say "Teachers have expert knowledge", "Experiments are important in science", or "Dogs are popular pets".
§ If you're discussing an uncountable noun in a general sense, leave out the article. For example, you might write "Love is a wonderful feeling", or "Health is something that concerns us all".
§ A few other kinds of nouns never take an article. Names of languages and nationalities, names of academic subjects, and names of sports are examples of these.
