Cerniera vs. cardine
While both words can translate to hinge in English, they describe different mechanical actions and carry distinct figurative weights. "Cerniera" is a general purpose joint or fastener, whereas "cardine" refers to a heavy load-bearing pivot.
Cerniera
B1This is the most common word for a mechanical hinge found on furniture, cabinets, windows, and interior doors (consisting of two plates and a pin). Crucially, "cerniera" also means zipper used on clothing.
La cerniera dell'anta dell'armadio si è allentata.
(The hinge of the wardrobe door has come loose.)
Dobbiamo mettere un po' d'olio sulla cerniera perché cigola.
(We need to put some oil on the hinge because it squeaks.)
Ho rotto la cerniera della mia giacca preferita.
(I broke the zipper of my favorite jacket.)
Queste cerniere sono fatte di ottone resistente.
(These hinges are made of durable brass.)
Cardine
C1Technically, this refers to the male pivot or vertical pin typically used for heavy gates, exterior shutters, or ancient doors. Figuratively, it is very commonly used to mean a cornerstone, pillar, or key point of a theory or argument.
Il vecchio cancello di ferro pesante ruota su un solo cardine.
(The old heavy iron gate rotates on a single hinge pivot.)
La libertà di espressione è un cardine della democrazia.
(Freedom of speech is a cornerstone of democracy.)
Il fabbro ha dovuto sostituire i cardini arrugginiti del portone.
(The blacksmith had to replace the rusted pivots of the main door.)
Questo principio è il cardine di tutta la sua filosofia.
(This principle is the pivotal point of his entire philosophy.)
Summary
Use "cerniera" for standard hardware found on kitchen cabinets, light doors, or when talking about the zipper on your pants. Use "cardine" when referring to the heavy pivot of a garden gate or, figuratively, when describing the fundamental cornerstone of an idea.