I tried to change his mind but couldn’t. (Talking about the past) I’ve tried to change his mind but haven’t succeeded. (Talking about the present results of past events) Mixing tenses as your example does sounds a little less precise, but not bad enough to be wrong.

Francesca, let me try to explain this by making a comparison with your native Italian. Present Perfect in English is very different from Italian. The Italian version truly is a tense (dealing with timeframe), but the English version is not. The Italian version, unlike the English, can place an action in the past. The English version lacks this capacity. The only time information it provides is that which is given by the present tense of the verb "to have". It makes sense in Italian to say "Ieri sono andato al parco e lì ho mangiato del cioccolato". In English, this would be nonsense: "Yesterday I have gone to the park and there I have eaten some chocolate". The reason it is nonsense is that "I have gone" means "I now have experience of going", and "I have eaten" means "I now have experience of eating". Therefore, in English this would say "Yesterday, I now have the experience of going to the park and there I now have the experience of eating some chocolate." Do you see how strange that might sound to us? The sentence starts in the past with "yesterday", but jumps to the present "I now have". It would be just as strange as saying "Yesterday I am happy right now". Your original sentence means "I now have the experience of trying to change his mind, but I did not succeed." This sentence has no timeline problem. The sentence starts by telling an experience that you now have, and finishes by telling what happened in the past. A simpler example would be "I have eaten chocolate, but I did not like it". It is crystal clear.

It's absolutely fine as it is. I would happily say it like that in a normal conversation.

It needs no improvement. It is perfect. When you say it your way it means: This happened: I did not succeed Your experience was: Trying to change his mind ("trying" is a noun here) You could reverse the tenses and instead say "I tried to change his mind, but I have had no success." When you say it that way it means This happened: I tried to change his mind Your experience was: Having no success ("having" is a noun) You could also use Present Perfect twice or Simple Past twice. The issue is not which choice is better. Rather, the issue is what you want to say. Do you want to talk about your experiences or things that happened. There is subtle but real difference! I find it useful to think of the Present Perfect tense as a way to talk about experiences. "I have PAST PARTICIPLE" means the same as "I have experience of PRESENT PARTICIPLE". "I have tried" means the same as "I have experience of trying".

It's correct but if you wanna make it sound more natural use : I couldn't !!! I've tried to change his mind, but I couldn't!! You can use { I didn't succeed } but it sounds kinda formal !!!!