When you mean ALL boys (or ALL milk, or ALL girls, etc.), you do not need an article. Or, when you mean boys IN GENERAL (milk IN GENERAL, girls IN GENERAL, etc.), you do not need an article. If you mean only one or a specific group of boys (specific milk, specific girls, etc.), then you must use an article. "Milk has important vitamins and minerals." That is, "ALL milk has important vitamins and minerals." But ... "The milk has gone bad." This means that SPECIFIC milk (in my refrigerator) is spoiled, not ALL milk in the world. I need an article. "Girls grow faster than boys." That is "GIRLS IN GENERAL grow faster than BOYS IN GENERAL." But ... "The girls are chasing the boys." Now, SPECIFIC girls (which I see) are chasing SPECIFIC boys (which I see), not ALL girls in the world chasing ALL boys.

As a beginner you don't need to study articles first. basic rules: "no article"(zero article): 1) plural nouns in general e.g. Boys run faster than girls. (a general statement) 2) singular uncount nouns - nouns which you cannot count (e.g. music, milk, etc.) x a - with singular count nouns beginning with a vowel (pronounced vowel - e.g. car (c - is a vowel) an - with singular count nouns beginning with a consonant (pronounced consonant) - e.g. an apple vowels are: a, e, i, o, u countable x uncountable nouns countable - you can count them (e.g. one apple, two apples, etc.) uncountable - you can't count them (e.g. you cannot say "one music, two etc.) Find out more in Google or a good reference book for grammar is: Murphy, R.: Essential Grammar in Use, Oxford University Press. Search for "indefinite article" and "zero article".