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"Necesito" is a form of "necesitar", a transitive verb which is often translated as "to need". "Hay que" is a phrase which is often translated as "one must". Learn more about the difference between "necesito" and "hay que" below.
necesitar(
neh
-
seh
-
see
-
tahr
)
A transitive verb is a verb that requires a direct object (e.g. I bought a book.).
transitive verb
1. (to require)
a. to need
Necesito canela para la tarta.I need some cinnamon for the cake.
An auxiliary verb, or helper verb, is a conjugated verb that comes before a main verb and determines the main verb's tense, mood, or aspect (e.g. I have gone.).
auxiliary verb
2. (to be necessary)
a. to need
Necesitan salir ya si quieren alcanzar su vuelo.You need to leave now if you want to make your flight.
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hay que(
ay
 
keh
)
A phrase is a group of words commonly used together (e.g once upon a time).
phrase
1. (general)
a. one must
Hay que ser valiente de cara al mal.One must be brave in the face of evil.
b.
This refers to an idiomatic word or phrase for which there is no word-for-word translation.
no direct translation
Hay que llenar la solicitud a mano.The application must be filled out by hand.
Hay que trabajar mucho para ser exitoso.You have to work hard to be successful.
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