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"Regardless" is an adverb which is often translated as "a pesar de todo", and "whether" is a conjunction which is often translated as "si". Learn more about the difference between "regardless" and "whether" below.
regardless(
rih
-
gard
-
lihs
)An adverb is a word that describes a verb, an adjective, or other adverbs (e.g., to run quickly, very tired).
a. a pesar de todo 
This is a setback, but I will achieve my dreams regardless.Esto es un contratiempo, pero realizaré mis sueños a pesar de todo.
b. pase lo que pase 
He will always be a criminal, regardless.Siempre será un criminal, pase lo que pase.
c. en cualquier caso 
Regardless, he will eat as much as he pleases.En cualquier caso, él comerá tanto como le plazca.
whether(
weh
-
thuhr
)A conjunction is a word that connects words, phrases, clauses, or sentences (e.g., The cat and the dog slept.).
a. si 
I don't know whether I am going to do it today or tomorrow. No sé si lo voy a hacer hoy o mañana. 
a. si 
I'll try to find out whether she's at the office.Voy a tratar de averiguar si está en la oficina.
a.  no direct translation
This refers to an idiomatic word or phrase for which there is no word-for-word translation.
Alicia has made up her mind - she'll resign, whether or not she gets this other job.Alicia se decidió: va a renunciar, consiga o no el otro trabajo.
Whether you like it or not, I will buy this shirt.Te guste o no, me voy a comprar esta camisa.