Some references to a thing represented by a noun point to, in the abstract, all of that class, and in such cases Spanish uses a definite article, where English does not (left-hand column below).

Others make reference only to a part of the class, referring only to the amount of milk that one or more persons might drink, all of the history one might study or teach, etc. In some such cases, an amount is expressed, or the indefinite "some" or "any" is implied. In these cases the definite article is not used.

• La leche es blanca.
  • Milk (all milk) is white.
• El flan es un postre popular. Me encanta el flan.
  • Flan is a popular dessert. I love flan.
• La historia es tan importante como la sociología.
  • History is as important as sociology.
• Los profesores trabajan en las universidades.
  • Professors work in universities.

• Tomo (mucha) leche.
  • I drink (a lot of) milk.
• Siempre pedimos flan cuando salimos a comer.
  • We always order flan when we go out to eat.
• Estudio historia pero enseño sociología.
  • I am studying history but I teach sociology.
• Hay profesores que no dan clases.
  • There are (some) professors who don't teach (any) classes.