I heard an American say « I lost my keys » but he didn’t have them when he said that.
I known that in British English, I lost mean I lost in the past but I have found it now and, that I have lost means that I don’t have found them.
So can you explain me about the difference in American English and if you can give me some other examples ?
knownknow that in British English, "I lost" means I lost something in the past, but I have found it now, and [no comma] that "I have lost" means that I don’t have found them.I haven't found it.
Note above how to write this sentence.
Where did you get this information? I have never heard anything like this about the use of "lose" in British English. I don't think the person who told you this knows what he's talking about.
The British tend to use the present perfect more often than Americans do, but the meanings of the tenses are the same in either case. The present perfect indicates that the speaker thinks what is said has current relevance. Americans are less likely to think that events they report have current relevance, so they sometimes use the simple past where the British might use the present perfect. All totaled up, the differences are miniscule.
When a child approaches the dinner table, a British mother may ask "Have you washed your hands?" while an American mother may ask "Did you wash your hands?" The practical meaning is the same either way.
I can't imagine any cases where this alternation between tenses would be so different that American people could not understand British people, or vice versa. This means that, as a foreigner who is learning English, there is no reason to panic over small differences like this. Besides, I see from the way you phrased your question above that there are plenty of other, more important aspects of English you might focus on for now.