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"Garantizar" is a transitive verb which is often translated as "to guarantee", and "asegurar" is a transitive verb which is often translated as "to assure". Learn more about the difference between "garantizar" and "asegurar" below.
garantizar(
gah
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rahn
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tee
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sahr
)A transitive verb is a verb that requires a direct object (e.g., I bought a book.).
1. (commerce)
a. to guarantee 
El concesionario garantiza sus coches durante cinco años.The car dealership guarantees its cars for five years.
b. to warrant  (formal) (United States)
A word or phrase used to refer to the second person formal “usted” by their conjugation or implied context (e.g., usted).
Regionalism used in the United States
Garantizamos todos nuestros productos durante un período de dos años.We warrant all our products for a two-year period.
2. (to promise)
a. to guarantee 
Todo saldrá bien; te lo garantizo.Everything will work out all right; I guarantee it.
a. to ensure 
¿Cómo podemos garantizar que esto no vuelva a ocurrir?How can we ensure that this doesn't happen again?
4. (to endorse)
a. to vouch for 
No puedo garantizar la profesionalidad de un desconocido ante mi jefe.I can't vouch for the professionalism of someone I don't know to my boss.
5. (finance)
a. to act as guarantor for 
Mis padres garantizaron el préstamo de mi carro.My parents acted as guarantors for the loan for my car.
b. to stand surety for 
No te recomiendo garantizar la deuda de tu primo.I wouldn't recommend standing surety for your cousin's debt.
asegurar(
ah
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seh
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goo
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rahr
)A transitive verb is a verb that requires a direct object (e.g., I bought a book.).
1. (to promise)
a. to assure 
Te aseguro que nos vamos a divertir en la fiesta.I assure you that we are going to have fun at the party.
b. to guarantee 
El alcalde aseguró que los impuestos se reducirían.The mayor guaranteed that taxes would be lowered.
2. (to fasten)
a. to secure 
Aseguramos la puerta con cuatro cerraduras para estar seguros.We secured the door with four locks to be safe.
a. to insure 
Siempre aseguro mi teléfono celular en caso de que lo pierda.I always insure my cellphone in case I lose it.
asegurarse
A reflexive verb is a verb that indicates that the subject performs an action on itself (e.g., Miguel se lava.). 
4. (to ensure)