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Incendio vs. fuego

The words incendio and fuego are often used interchangeably in Spanish to refer to a fire, but there are nuances between their uses.

Incendio

B1
Incendio is used primarily to describe uncontrolled, large scale and often disastrous fires. This term is typically used when referring to wildfires or building fires.
El incendio en el bosque está fuera de control.
(The forest fire is out of control.)
El incendio del edificio causó grandes pérdidas.
(The building fire caused great losses.)

Fuego

A1
Fuego is a more general term for fire. It can be used metaphorically or literally and does not necessarily imply destruction. However, it can also refer to dangerous, uncontrolled fires in some contexts.
¡Por favor, apaga el fuego!
(Please, put out the fire!)
Jugar con fuego es peligroso.
(Playing with fire is dangerous.)
El fuego en la chimenea me tranquiliza.
(The fire in the fireplace relaxes me.)

Summary

Incendio and fuego both refer to a situation where something is burning. However, incendio is more specific to uncontrolled, often large-scale and disastrous fires such as wildfires or building fires. On the other hand, fuego is used as a general term for any type of fire and it can also be used metaphorically.