How to Say "in vain" in Spanish
The Spanish word for “in vain” is “balde” — A1 level. This is a very common word in everyday Spanish.

Examples
El niño lleva un balde azul para recoger arena en la playa.
The boy is carrying a blue bucket to collect sand at the beach.
Todo mi esfuerzo fue en balde; no logré convencerlo.
All my effort was in vain; I didn't manage to convince him.
En esta tienda, si compras dos camisas, la tercera te sale de balde.
In this store, if you buy two shirts, the third one is for free.
Bucket vs. Free vs. Vain
When 'balde' stands alone, it's a bucket. But when you add 'de' or 'en' before it, the meaning changes completely to 'for free' or 'in vain'.
The 'De' and 'En' rule
To say something is free, use 'de balde'. To say something was a waste of time, use 'en balde'. These phrases never change their ending, even if the thing you are talking about is plural.
Using 'balde' alone for 'free'
Mistake: “La entrada es balde.”
Correction: La entrada es de balde (or 'gratis'). You must use 'de' to give it the meaning of 'free of charge'.
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