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

Examples
Yo advertí a mi jefe sobre el error antes de que fuera público.
I warned my boss about the error before it went public.
Ella me dijo que tuviera cuidado, pero yo no advertí el peligro.
She told me to be careful, but I didn't see the danger. (Note: Here, 'no advertí' means 'I didn't notice/realize the danger'.)
El guardia civil me advertí de que no podía aparcar allí.
The police officer warned me that I couldn't park there.
Using 'Advertir' with 'de'
When you warn someone about a specific thing (a noun), you often need to use 'de' (of/about): 'Advertí a María del ladrón' (I warned Maria about the thief).
The Stress of 'Advertí'
'Advertí' is stressed on the last syllable (the 'í'). This is typical for the 'yo' form in the simple past (preterite) tense for -er and -ir verbs.
Forgetting the stem change
Mistake: “Using 'adverto' instead of 'advierto' in the present tense.”
Correction: The vowel 'e' changes to 'ie' in most present forms: 'Yo advierto' (I warn).
Related Translations
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