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"A" is a preposition which is often translated as "to", and "tranquilo" is an adjective which is often translated as "calm". Learn more about the difference between "a" and "tranquilo" below.
a, la a(
ah
)A preposition is a word that indicates the relationship between a noun and another word (e.g., He ran through the door.).
a.  no direct translation
This refers to an idiomatic word or phrase for which there is no word-for-word translation.
Podía respirar a duras penas.He was hardly able to breathe.
a.  no direct translation
This refers to an idiomatic word or phrase for which there is no word-for-word translation.
¡A fregar los platos!Go do the dishes!
¡A la cama!Off to bed!
a.  no direct translation
This refers to an idiomatic word or phrase for which there is no word-for-word translation.
Me encantaría ganar dinero a espuertas.I would love to earn lots of money.
Llovía a cántaros.It was pouring.
A feminine noun is almost always used with feminine articles and adjectives (e.g., la mujer bonita, la luna llena).
An abbreviation is the shortened form of a word or group of words (e.g., "UFO" = "unidentified flying object"; "p." = "page").
a. A 
El circuito transporta una corriente de 1 A.The circuit carries a current of 1 A.
La intensidad de la corriente eléctrica se mide en A.The intensity of an electric current is measured in A.
tranquilo(
trahng
-
kee
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loh
)An adjective is a word that describes a noun (e.g., the big dog).
1. (serene)
3. (carefree)
An adverb is a word that describes a verb, an adjective, or other adverbs (e.g., to run quickly, very tired).
4. (colloquial) (at least) (Mexico)
A word or phrase that is commonly used in conversational speech (e.g., skinny, grandma).
Regionalism used in Mexico
An interjection is a short utterance that expresses emotion, hesitation, or protest (e.g., Wow!).
d. chill  (colloquial)
A word or phrase that is commonly used in conversational speech (e.g., skinny, grandma).
¡Tranquilo, hombre! Llegaremos.Chill, man! We'll get there.