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"No" is a form of "no", an adverb which is often translated as "no". "Si no" is a conjunction which is often translated as "unless". Learn more about the difference between "si no" and "no" below.
si no( 
see
noh
)A conjunction is a word that connects words, phrases, clauses, or sentences (e.g., The cat and the dog slept.).
1. (except if)
a. unless 
No pueden pasar a la sala de conferencias si no son prensa.You cannot go into the conference room unless you are press.
a.  no direct translation
This refers to an idiomatic word or phrase for which there is no word-for-word translation.
Si no cambia de actitud, va a tener muchos problemas en la vida.If he doesn't change his attitude, he's going to have a lot of trouble in life.
Si no fuera por Merche, nunca lo hubiéramos conseguido.If it weren't for Merche, we'd never have managed it.
b.  no direct translation
This refers to an idiomatic word or phrase for which there is no word-for-word translation.
Si no nos hubieran ocultado información, podríamos haber hecho algo al respecto.Had they not hid information from us, we could have done something about it.
Hace tiempo que hubiera dejado mi puesto si no fuera por el profundo respeto que siento hacia el Presidente.I would have resigned my post long ago if it weren't for my profound respect for the president.
3. (or)
a. otherwise 
Prueba a llamarlo por teléfono; si no, siempre puedes mandarle un email.Try calling him up; otherwise, you can always send him an email.
b. or else 
Tendrás que ayudarme en la casa; si no, ya puedes despedirte de un celular nuevo estas Navidades.You'll have to help me with the house chores; or else, you can forget about getting a new cellphone for Christmas.
no(
noh
)An adverb is a word that describes a verb, an adjective, or other adverbs (e.g., to run quickly, very tired).
1. (negation)
a. no 
¿Te gustó el concierto? - No, ese estilo de música no me gusta.Did you like the concert? - No, I don't like that style of music.
c. non- 
Una organización no gubernamental es una asociación que actúa con independencia del gobierno.A non-governmental organization is an association that operates independently from the government.
a.  no direct translation
This refers to an idiomatic word or phrase for which there is no word-for-word translation.
Vamos a la fiesta esta noche ¿no?We're going to the party tonight, aren't we?
Firmaste el contrato ¿no?You signed the contract, didn't you?
a.  no direct translation
This refers to an idiomatic word or phrase for which there is no word-for-word translation.
Deberíamos llamarlos ¿no?We should call them, shouldn't we?
Te acompaño ¿no?Why don't I come with you?
An interjection is a short utterance that expresses emotion, hesitation, or protest (e.g., Wow!).
a. no 
Quiero que limpies tu cuarto ahora mismo. - ¡No!I want you to clean your room right now. - No!