Why do we put *the to uncountable things like air or sky?
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The usage of articles with a given noun in any given case is determined by the meaning expressed by that noun, i.e., by the thing to which the noun refers (called the “referent” — 指示対象・指示物) in that case. Thus, in general, we cannot say that a given noun itself is countable or uncountable. This is a case-by-case matter that is determined by how the noun is used.
The meaning expressed by a noun is affected by both the expressions modifying the noun and the context within which the noun appears. Let us refer to the meaning that a noun expresses without modifiers and within a neutral context as its “base” meaning. Then, let us consider a noun that is uncountable in its base meaning. “Air” is such an example. In the following sentence, “air” is used with its base meaning:
- Air is composed mainly of nitrogen and oxygen.
Here, because there is no information that limits the meaning expressed by “air,” it is being used in its base meaning, and hence is uncountable.
Next, let us consider the following sentence:
- The air in this room is quite warm.
Here, “air” is modified by the prepositional phrase “in this room.” As a result, within this sentence, this noun refers to a particular realization of this substance, and for this reason, it becomes a countable noun.
For detailed discussion, see 『科学論文の英語用法百科＜第２編＞』