Basic Spanish Grammar
Spanish language
Quick Spanish Reference for grammar

Talking in general a definite article is used in Spanish whenever "the" is used in English. However there are some exceptions.

1. With abstract nouns: La pobreza es un grave problema. (Poverty is ...)
2. With most titles of people: el señor López abrió la conferencia (Mister López...), el presidente Rodríguez Zapatero, la doctora García...
3. With infinitives used as nouns, specially if they are the subject of the sentence: El practicar deporte es bueno para la salud (Practising sports... )
4. With nouns listed in a series: Tengo el libro, el cuaderno y el diccionario (I have the book, the notebook and the dictionary)
5. With noun of weight an measure: Las naranjas cuestan 3 Euros el kilo. (The oranges cots 3 Euros per kilo)
6. With days of the week: El curso de español empieza el lunes. (The Spanish course starts on Monday)
7. When telling the time. Son las doce. (It is twelve o'clock)
8. With parts of the body or clothes, specially if the possessor is clearly identify, as in case of reflexive verbs. Voy a lavarme las manos (I am going to wash my hands)
9. To generalize. Los españoles se acuestan tarde. (Spanish people... )

Un, una unos, unas (a, some) normally precede a noun.
To say the profession with the verb to be, there is no article: Soy profesora ( I am a teacher).