Bunu İngilizce (Birleşik Krallık) da nasıl dersiniz? tempo fa non lo avrei mai detto
tempo fa non lo avrei mai pensato.
my point is : "I would do it" and "I would have done it" are interchangeable for describing past action?

Bir çeviriye bakın

They are not really interchangeable.

You can sometimes use the present tense to talk about the past in English. This is called the "historic present".

But "I would do it" has more than one meaning. It could be a conditional sentence, with the condition not included or stated elsewhere in the writing or speech. Or it could be a statement of a habitual past action. It could be a statement of desire or wish. Or it could be a contrast to another person; "She couldn't take the decision, but I would do it."

"I would have done it" is a more straightforward past conditional (or "conditional perfect".) Typically this is used in response to another part of a sentence or paragraph, and suggests that if circumstances had been different, I would have done something.

So you can see an area of overlap. But these are not really interchangeable.

https://dictionary.cambridge.org/dictionary/eng...
https://www.lexico.com/definition/historic_present

https://www.englishclub.com/grammar/verbs-modal...
https://www.lexico.com/en/definition/would

https://www.ef.co.uk/english-resources/english-...
https://www.myenglishpages.com/site_php_files/g...