Such as "I would not have done that."

"Mi ne estus farinta tion". Or you could shorten it: "Mi ne farintus tion".

Bear in mind however that Esperanto generally prefers simplicity. If the context is clear enough in setting the conditional into the past it's quite alright to say "Mi ne farus tion".

This must have been discussed like a thousand times, and I probably know what sudanglo will respond to my reaction here. Anyhow:

All you need is -us: "Mi ne farus tion."

If you learn e.g. Polish, you will see that this is fine and enough in > 95% of the instances, and if context isn't clear, you can add something like jam, hieraŭ etc.

estus X-inta does NOT express that something would have been in the past, it just says that X is finished at the time of estus (like estis/as/os X-inta) - which may be in the future.
But speakers of western European seemingly don't want to accept that Esperanto lacks a distinction here they're used to make, so they misuse estus X-inta in order to express conditional past.

tommjames:
Bear in mind however that Esperanto generally prefers simplicity. If the context is clear enough in setting the conditional into the past it's quite alright to say "Mi ne farus tion".

inb4 sudanglo disagreeing, leading to this thread becoming at least 3 pages long

So long as Sudanglo's expected three pages are three quality pages, I rather look forward to them.

Kirilo81:it just says that X is finished at the time of estus

The very idea of "estus time" just sounds wrong to me. We have verb tenses to express the "time", and we have verb moods to express other things. In this case conditional mood implies, that the action takes place in some hypothetical reality, if you which, but not on some particular time.

Using just -us may be ambiguous in many cases. For example:
Vi diris fivortojn hieraŭ. Mi deziras, ke vi ne farus tion.
Do I want him not to say the fi-words in the future? Or do I want that he wouldn't have said those words in the past?

Vi diris fivortojn hieraŭ. Mi deziras, ke vi ne estus farinta tion.
This one is much more precise, but a little bit more ugly as well.

In most cases I would just rephrase the though in entirely different words. Something like:
Vi diris fivortojn hieraŭ. Ne decis/indis/taŭgis fari tion.

Kirilo81:estus X-inta does NOT express that something would have been in the past, it just says that X is finished at the time of estus (like estis/as/os X-inta) - which may be in the future.
But speakers of western European seemingly don't want to accept that Esperanto lacks a distinction here they're used to make, so they misuse estus X-inta in order to express conditional past.

Incorrect. See this thread.

Vilius:Vi diris fivortojn hieraŭ. Mi deziras, ke vi ne farus tion.
Do I want him not to say the fi-words in the future? Or do I want that he wouldn't have said those words in the past?

I tend to (perhaps wrongly, I fully admit) think of things this way:

I wish you wouldn't do that.
Mi deziras, ke vi ne farus tion.
I don't interpret this as being ambiguous in time, but rather interpret the tense as being set by deziras. While this may not be technically accurate, it just seems to be my instinctive interpretation. If I were the one saying that sentence, my meaning would be entirely about the present.

I wish you wouldn't have done that.
Mi deziras, ke vi ne farintus tion.
I get that the -intus construction is pretty hideous. But it's convenient, and here it takes the present tense of the last sentence and puts the unfortunate action clearly into the past (while still indicating that the wishing is being done in the present).

Like I said, I might not be totally right. But, until now (at least), that's how I would interpret those sentences, with the -us at times taking the tense of the other (tensed) verb, and at other times modified to show a different time.

Vilius:

Kirilo81:it just says that X is finished at the time of estus

The very idea of "estus time" just sounds wrong to me. We have verb tenses to express the "time", and we have verb moods to express other things. In this case conditional mood implies, that the action takes place in some hypothetical reality, if you which, but not on some particular time.

Using just -us may be ambiguous in many cases. For example:
Vi diris fivortojn hieraŭ. Mi deziras, ke vi ne farus tion.
Do I want him not to say the fi-words in the future? Or do I want that he wouldn't have said those words in the past?

Vi diris fivortojn hieraŭ. Mi deziras, ke vi ne estus farinta tion.
This one is much more precise, but a little bit more ugly as well.

In most cases I would just rephrase the though in entirely different words. Something like:
Vi diris fivortojn hieraŭ. Ne decis/indis/taŭgis fari tion.

It may be just me, but I would say, "Mi dezirus, ke vie ne faru tion."

tommjames:

Kirilo81:estus X-inta does NOT express that something would have been in the past, it just says that X is finished at the time of estus (like estis/as/os X-inta) - which may be in the future.
But speakers of western European seemingly don't want to accept that Esperanto lacks a distinction here they're used to make, so they misuse estus X-inta in order to express conditional past.

Incorrect. See this thread.

Ort correct, see the same thread. My arguments are still valid, '-intus' as past irrealis is neither Zamenhofian nor useful nor desireable.
IMHO it's comparable to 'far', and I hope it'll disappeare just like it.