The phrase "I have warned" is correct and usable in written English. You can use it when you want to show that you have previously warned someone of a potential danger or issue. For example: "I have warned you multiple times to be careful when you close the door or you will break the window.".
This is what I have warned my children about so many times.
Ok, If you don't want to pay, it's your life but I have warned you".
The tape's reference to Australia said: "I have warned the Australian people.
But I have warned the missionary he must not force Christianity on the people if they do not want it".
My mother became nervous then, warning you not to go there, as she had so often warned me, as I had warned you the night you came, but your parents did not share her concern.
"John, I've warned you about this," I said.
"I've warned people, make up your mind".
When I feel like I can't trust my brain 100%, Ludwig really comes in handy. It makes me translate and proofread faster and my output more reliable.
Claudia Letizia
Head Translator and Proofreader @ organictranslations.eu