The word "the" occupies a unique place in the Englishlanguage as the only word that grammarians classify as a definitearticle. It's not quite so simple in Spanish, where the English "the"has four equivalents. Like most adjectives, the definite article inSpanish varies with number and gender:

• Singular masculine: el
• Singular feminine: la
• Plural masculine: los
• Plural feminine: las

Although there are a few exceptions, as a general rule a definitearticle is used in Spanish whenever "the" is used in English. ButSpanish also uses a definite article in many cases where English doesnot. Although the following list isn't exhaustive, and there areexceptions some of these rules, here are the major instances whereSpanish includes a definite article absent in English:

With abstract nouns and nouns used in a general sense:In English, the article is often omitted with abstract nouns and nounsthat refer more to a concept than a tangible item. But it still isneeded in Spanish. A few examples might help clarify: La ciencia es importante. (Science is important.) Creo en la justicia. (I believe in justice.) Estudio la literatura. (I study literature.) La primavera es bella. (Spring is beautiful.)

With most titles of people: The definite article is used before most titles of a person being talked about. El presidente Bush vive en la Casa Blanca. (President Bush lives in the White House.) Voy a la oficina de la doctora González. (I'm going to the office of Dr. Gonzalez.) Mi vecina es la señora Jones. (My neighbor is Mrs. Jones.) The article is omitted, however, when directly addressing the person. ¿Cómo está usted? profesora Barrera. (How are you, Professor Barrera?)

Before days of the week: Days of the week are always masculine. Except in constructions where the day of the week follows a form of ser (a verb for "to be"), as in hoy es martes (today is Tuesday), the article is needed. Vamos a la escuela los lunes. (We go to school on Mondays.) El tren sale el miércoles. (The train leaves on Wednesday.)

Before verbs used as subjects: In Spanish, infinitives (the basic form of a verb) can be used as nouns. The article el is used when one is used as the subject of a sentence. El escribir es difícil. (Writing is difficult.) El esquiar es peligroso. (Skiing is dangerous.) No me gusta el nadar. (I don't like swimming. In Spanish, this sentence has an inverted word order that makes nadar the subject.)

Often before names of languages:The article generally is used before names of languages. But it can beomitted immediately following a verb that is used often with languages,such as hablar (to speak), or after the preposition en. El inglés es la lengua de Belice. (English is the language of Belize.) El alemán es difícil. (German is difficult.) Hablo bien el español. (I speak Spanish well.) But, hablo español. (I speak Spanish.) No puede escribir en francés. (He can't write in French.)

With some place names:Although the definite article is seldom mandatory with place names, itis often used with many of them. The list of place names using thedefinite article is quite arbitrary and can be found here. La Habana es bonita. (Havana is pretty.) La India tiene muchas lenguas. (India has many languages.)

With nouns joined by "and": In English, it often isn't necessary to include the "the" before each noun in a series. But Spanish often requires it. La madre y el padre están felices. (The mother and father are happy.) Compré la silla y la mesa. (I bought the chair and table.)

Note: Spanish also has a neuter definite article, lo, explained in this lesson. It has specific uses different than most of the ones listed on this page.

For an eye-popping introduction to the future of lighting, walk into the lobby of Philips Color Kinetics' headquarters outside Boston. The reception area is bathed first in pink, then lavender, then pink again. Hallways on either side undulate with a tunnel of pulsing blue, then purple, lights. The surrounding walls display Philips' latest products as they have been installed in a rotating selection of buildings, including the CN Tower in Toronto and Harrah's casino in Atlantic City, N.J. All this lighting is done with light-emitting diodes, or LEDs.

The energy-hogging incandescent lightbulb as we know it got a death sentence when President Bush signed the Energy Independence Act in December 2007; most such bulbs will be phased out beginning in 2012. Europe is nixing incandescents beginning in 2009. The race is on to develop devices that do a better job of converting electricity to light. Compact fluorescent bulbs have become a popular alternative, but LEDs have the potential to be twice as efficient. They typically last 50,000 hours, compared with 10,000 for a compact fluorescent and 1,000 for an incandescent bulb.

The hurdle so far is price. LEDs, semiconductor diodes that light up when current runs through them, are 10 to 20 times as expensive as incandescents, too high for use at home. But the Department of Energy predicts that steady improvements in semiconductor manufacturing will bring the price of white LEDs below that of compact fluorescents in five years. Large consumers of electricity see a quicker payback. Wal-Mart Stores (nyse: WMT - news - people ) and Shaw's have begun putting LEDs in some freezer cases. Lansing, Mich. is testing LED street lights. "The consumer is the last to convert," says Charles Jerabek, chief executive of Osram Sylvania, Siemens (nyse: SI - news - people )' North American lighting division. LEDs account for only 12% of the unit's $2.5 billion in revenue, but it's the fastest-growing piece.

At Osram Sylvania labs in Danvers, Mass. researchers are experimenting with blankets woven with LEDs to treat babies with severe jaundice. They're also developing LEDs for use in endoscopic surgery to replace the xenon lights currently in use, which are bulky and don't last long. Another innovation: square tiles of light that could replace fluorescent office ceiling lighting.

Twenty miles away in the showroom at Philips Color Kinetics a "screen" made of 50 vertical tubes (each 2 inches wide and 4 feet tall and composed of 760 individual diodes) of programmable LEDs projects a parrot moving in a forest, then flames flickering in a fireplace. By feeding video input into a computer, Philips makes the LED tubes function as a low-resolution video screen. Since LED lights can be tuned to different wavelengths of color, Philips is also working with retailers of luxury goods and jewelry to choose the best color for showcasing each product. Diamonds look best under blue-tinged white light, while gold shows better in warmer tones.

ElectroniCast Consultants of Upper Lake, Calif. says the market for LEDs in lighting should grow to $1.4 billion in 2009. "As the price continues to drop and the performance continues to increase, we'll see more adoption," says Philips Color Kinetics' vice president of innovation Kevin Dowling. "We haven't scratched the surface of this technology."