When a feminine noun begins with a stressed "a" or "ha", the definite article "el" is used instead of "la", ONLY if the definite article is placed next to the noun. Notice how this rule does not apply to plural nouns.
- el hacha (the axe), las hachas
- el agua (the water), las aguas
- el ala (the wing), las alas
- el arma (the weapon), las armas
When the article is not next to the noun, the article reverts to "la":
- La terrible arma de destrucción masiva.
(the terrible weapon of mass destruction.)
- La única hacha que tengo conmigo.
(the only axe I have with me.)
Keep in mind that ONLY nouns follow this rule. In the case of adjectives there is no change:
- Ella pertenece a la alta sociedad. (She belongs to the high society.)
- La hermosa águila americana. (The beautiful American eagle.)
Remember that definite articles are followed by nouns or adjectives (or words that function as such), and cannot be followed by verbs or indefinite pronouns such as "algún", "ningún", "cualquier".