In English, unlike Spanish, uses the indefinite article before the generic predicate. The article plays its essential role, which is to singularize an object among the class designated by the noun, which is a possibility that does not exist in English, since it uses the article in both cases. The individualization becomes apparent, and in Spanish the article becomes necessart, when the noun is used with an adjective.
It is not used in Spanish when it is used in the English text before “hundred”, “thousand”, “certain”, “such”, “what”, “as” and “of”.
Another point of divergence: the omission of the article in English when Spanish uses the definite article.
It also arises in situations with personal addresses (Dr., Mr., etc.), positions, and professional titles (when they are not used as appositives). It happens as well in prepositional expressions that precede nouns that indicate specific places or that have an adverbial function. Additionally, names of people and geographical places with an adjective have different uses in the two languages. In Spanish, all names that end in unstressed “a”‘s are feminine.
The article is not used in English before percentages, whereas in Spanish it is.
English uses the absence of an article with undetermined plural amounts often, where in Spanish the definite article is necessary.