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"Happy birthday" is a phrase which is often translated as "feliz cumpleaños", and "my darling" is a phrase which is often translated as "mi amor". Learn more about the difference between "happy birthday" and "my darling" below.
happy birthday( 
hah
-
pi
buhrth
-
dey
)A phrase is a group of words commonly used together (e.g., once upon a time).
1. (general)
a. feliz cumpleaños 
So, you're 18 today! Happy birthday!¡Así que cumples los 18 hoy! ¡Feliz cumpleaños!
my darling( 
may
dar
-
lihng
)A phrase is a group of words commonly used together (e.g., once upon a time).
1. (as a noun)
a. mi querido 
Mom, do you love me? - Of course I love you, my darling daughter.Mamá, ¿me amas? - Claro que te amo, mi querida hija.
Today is my darling son's first day of school.Hoy el el primer día de la primaria de mi querido hijo.