Reemplazar vs. sustituir vs. reponer

In Spanish, the verbs 'reemplazar', 'sustituir', and 'reponer' are often used interchangeably to convey the idea of taking something out and putting something else in its place. However, they can carry subtle differences in connotation and usage depending on the context.

Reemplazar

'Reemplazar' typically means to replace something with a substitute that is generally considered equivalent or better. It can also mean to take the place of someone or something.
Si se rompe tu teléfono, la compañía lo reemplazará sin costo.
(If your phone breaks, the company will replace it free of charge.)
El actor principal se enfermó y tuvo que ser reemplazado.
(The lead actor got sick and had to be replaced.)

Sustituir

'Sustituir' is similar to 'reemplazar' but often emphasizes the action of substitution, where one item or person takes the place of another, sometimes temporarily.
Puedes sustituir la leche de vaca por leche de almendras en esta receta.
(You can substitute cow's milk with almond milk in this recipe.)
El profesor titular está de licencia y será sustituido por un docente interino.
(The regular teacher is on leave and will be substituted by an interim teacher.)

Reponer

'Reponer' tends to imply replenishing or restoring something that has been used up, lost, or depleted. It focuses more on bringing back to a former state or level rather than introducing a new element.
Tuvimos que reponer el inventario después de las ventas navideñas.
(We had to replenish inventory after the Christmas sales.)
"Se me acabó el perfume, así que necesito reponerlo," dijo ella.
("I've run out of perfume, so I need to replace it," she said.)

Summary

While 'reemplazar', 'sustituir', and 'reponer' can all involve putting one thing in place of another in Spanish, 'reemplazar' is often used for complete replacements with an equivalent or better item, 'sustituir' emphasizes substitution potentially on a temporary basis, and 'reponer' focuses on restoring or replenishing what was there before.