How to Say "defenseless" in Spanish
The Spanish word for “defenseless” is “vulnerable” — B1 level. This is a very common word in everyday Spanish.

Examples
Los ancianos son un grupo especialmente vulnerable al frío.
Elderly people are a group especially vulnerable to the cold.
El sistema informático resultó ser vulnerable a un ataque sencillo.
The computer system turned out to be vulnerable to a simple attack.
Cuando te enamoras, te sientes muy vulnerable.
When you fall in love, you feel very vulnerable.
Adjective Agreement (Number)
Since 'vulnerable' ends in '-e', it doesn't change based on whether the noun is masculine or feminine (es: 'la zona vulnerable', 'el grupo vulnerable'). However, you must add an '-s' for plural nouns: 'zonas vulnerables' or 'grupos vulnerables'.
Using the wrong verb
Mistake: “Estar vulnerable”
Correction: Ser vulnerable. Because vulnerability is usually a defining characteristic or a long-term state, you almost always use 'ser' (to be) instead of 'estar' (to be, temporary state). 'Ella es vulnerable' (She is vulnerable) is correct.
Related Translations
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