Restar vs. sustraer
While both restar and sustraer can be translated as "to subtract" in English, they belong to different registers and have distinct secondary meanings. The word restar is the standard, everyday verb used for basic arithmetic, while sustraer is a more formal and technical synonym. Furthermore, restar is frequently used figuratively to mean downplaying or diminishing, whereas sustraer often translates to stealing or physically extracting something.
Restar
The verb restar is the most common and everyday way to say "to subtract" in basic math, and it is also widely used to describe diminishing, reducing, or taking away the value or importance of something.
Los estudiantes de primaria están aprendiendo a sumar y restar con bloques de colores.
(The elementary school students are learning to add and subtract with colored blocks.)
Si decides restar tres de ocho, el resultado final siempre será cinco.
(If you decide to subtract three from eight, the final result will always be five.)
Tenemos que restar todos los gastos fijos del presupuesto de este mes.
(We have to subtract all the fixed expenses from this month's budget.)
Ese pequeño error de pronunciación no debe restar valor a tu excelente presentación.
(That small pronunciation mistake should not subtract value from your excellent presentation.)
De ninguna manera quiero restar importancia a tus sentimientos durante esta difícil crisis.
(By no means do I want to downplay the importance of your feelings during this difficult crisis.)
Después de limpiar toda la casa, solo nos resta preparar la cena.
(After cleaning the whole house, it only remains for us to prepare dinner.)
Sustraer
The verb sustraer is a formal or technical term for "to subtract" in mathematics and accounting, but in everyday language it is far more commonly used to mean extracting, removing, or stealthily stealing something.
Para equilibrar la ecuación algebraica, debes sustraer la misma cantidad de ambos lados.
(To balance the algebraic equation, you must subtract the same amount from both sides.)
El contador procedió a sustraer los impuestos directamente del salario bruto del empleado.
(The accountant proceeded to subtract the taxes directly from the employee's gross salary.)
El profesor de física explicó cómo sustraer el peso del recipiente vacío antes de comenzar el experimento.
(The physics professor explained how to subtract the weight of the empty container before starting the experiment.)
El ladrón logró sustraer varias joyas muy valiosas de la caja fuerte del hotel.
(The thief managed to steal several very valuable jewels from the hotel safe.)
Los científicos utilizaron una jeringa especial para sustraer el líquido de la muestra biológica.
(The scientists used a special syringe to extract the liquid from the biological sample.)
Ningún ciudadano se puede sustraer del cumplimiento de las leyes de tránsito.
(No citizen can exempt themselves from complying with traffic laws.)
Summary
In short, use restar for everyday mathematical operations and situations where you want to express a reduction in importance, value, or simply what remains to be done. Reserve sustraer for highly formal mathematical, financial, and scientific contexts, or when you need to describe the physical act of stealing, extracting, or removing an object.