I found it interesting, when researching this topic, that the definition for the in Merriam-Webster’s online dictionary is nearly as long as this post — and that’s for just one set of functions for the word, as a definite article. The also functions, less often, as an adverb (“I like that one the best”) and, rarely, a preposition used in place of per (“Those cost ten dollars the dozen”).
And why should anyone feel the need to look the up in the dictionary? Isn’t it obvious? When it comes to meaning, yes, the role of the in a sentence is clear. But it’s not always clear whether the cast of characters in the sentence must include that role.
Consider the sentence “I looked out over the land.” The writer describes surveying a particular plot of land. However, “The price of land has gone down lately” omits the because no specific plot of land is being discussed; the topic is the concept of land in general.
But notice that in these nearly identical sentences, the difference in meaning seems to be the reverse of the difference in the previous paragraph: “I poured water out of the pitcher” explains what was poured, whereas “I poured the water out of the pitcher” emphasizes where the water came from.
Notice, however, that in those examples and the pair that follow, the is a marker for a second reference: “I put the shoes on and walked outside” emphasizes the particular pair of shoes, which presumably have already been referred to. “I put shoes on and walked outside” calls no special attention to the shoes; the sentence merely describes the writer’s routine preliminary to leaving the premises.
Sometimes the presence or absence or the in a sentence is irrelevant; the person quoted could have employed either usage: “She’ll have the strawberry cheesecake” identifies the particular dessert a diner wishes to be served, one either visible to the speaker or listed on a menu — a literal or implied second reference. “She’ll have strawberry cheesecake” means the same thing — with the subtle difference that the speaker is not directly alluding to the dessert selection visible in the form of a slice or a reference in text.
However, in the case of the pair of sentences about the shoes, the previous reference may be very important; these are magic shoes we’re reading about, for example. “I put shoes on and walked outside” presumably leaves the unusual footwear behind; “I put the shoes on and walked outside” moves the plot along.
The is deliberately omitted in many types of usage. For instance, most references to countries (“Afghanistan,” “Zimbabwe”) need no definite article, nor do references to their citizens (“Afghanis,” Zimbabweans”), unless, again, a particular subgroup is in question (“The Afghanis in the tour group kept to themselves”). Exceptions include use of “the Netherlands” and similar geographically influenced names. The same rule applies to names of other geographical or geopolitical features (“Mount Everest,” but “the Himalayas”; “Hawaii,” but “the Hawaiian Islands”; “Lake Tahoe”, but “the Great Salt Lake”).
Oddly, writers who would never make the mistake of omitting the before “Netherlands” or “Hawaiian Islands” frequently refrain from preceding names of organizations: “March of Dimes Foundation was founded in 1938.” Admittedly, some names do not merit the definite article, but they are usually obvious (“Project Reason,” “People for the American Way”). However, logic should override poor usage. The People for the American Way Foundation, associated with the organization named in the latter example, incorrectly self-identifies as “People for the American Way Foundation.” The rule of thumb is that any organizational name ending in a word referring to the entity (foundation, organization, project, etc.) requires the definite article, just as a generic reference such as “the foundation” merits it.