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"La cafeína" is a form of "cafeína", a noun which is often translated as "caffeine". "Estrés" is a noun which is often translated as "stress". Learn more about the difference between "la cafeína" and "estrés" below.
la cafeína()
A feminine noun is almost always used with feminine articles and adjectives (e.g., la mujer bonita, la luna llena).
el estrés()
A masculine noun is used with masculine articles and adjectives (e.g., el hombre guapo, el sol amarillo).
