mnlg:

Stefano B:I think it's important to be able to express an an going action that started some time ago and is still going on.

"Mi lernas esperanton de monato".

Which is also the simplest, and possibly most common form.

I didn't know you could use "de" in that sense.

But would "Mi estas estinta lernanta Esperanton de Monato" make sense? Doesn't that express exactly what the English present perfect continuous tense expresses?

I'm not seeing how one could get "I have been learning" from the words "Mi lernas". "Mi lernas" means "I learn" or "I'm learning", not "I have been learning".

I personally think it should be "Mi lernas esperanton ekde antaŭ unu monato."

At least, that's the way that I would put it, rightly or wrongly*. It states when you started, and it states that you are still in the process of doing it. The main difference between that and any of the more complex -inta or -anta forms is just that it's simpler. But it still means the same thing.

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* Or I might try and get clever and say something like "Mi lernas esperanton unu monato antaŭe". But only because it's shorter.

Stefano B:But would "Mi estas estinta lernanta Esperanton de Monato" make sense?

No, at least not to me. The double participle would make me think that there's a mistake somewhere. (also there is no need for a capital M).

Doesn't that express exactly what the English present perfect continuous tense expresses?

This I cannot say, since I am not a native speaker. But it is my humble opinion that this is not the right way to approach translation. You should use the tools Esperanto gives, rather than forcing it to mirror your own language's.

I suspect that *"I cleveram" would "express exactly" the Esperanto "Mi inteligentas". But "I cleveram" does not exist in English. I have to put it another way.

I'm not seeing how one could get "I have been learning" from the words "Mi lernas".

"I have been learning" means, if I am not mistaken, "my action of learning began some time ago and it is still ongoing". I think that "Mi lernas de ioma tempo" gives the same meaning, albeit roughly. You could put it another way, saying "Mi komencis lerni antaux ioma tempo kaj mi plu lernas nun".

It is completely acceptable to try and express the exact meaning you want to pass through. However sometimes you might decide to present the bulk of what you want to say with a particularly elegant form, leaving the details for later if they are really needed.

The complex tenses you are inquiring about exist in Esperanto, and were very well constructed by Zamenhof himself. In Esperanto, as in recent modern American English, these tenses are falling out of popular use. I learned them from the textbook, Teach Yourself Esperanto; my copy was published in the 1950's! I also bought and studied a pamphlet titled Ne Kiel Meier (Not like Meier) which showed that it is not proper to get all balled up in these forms. (Apparently the complex forms are even more complex in German?) There are occasions where the complex form sufices as no other can -- I just used one in a translation of the Gettysburg address -- "shall not have died in vain", ne estos mortintaj vane. Their death was accomplished in the past by the time the speaker spoke, but the speaker intended that their deaths will, in the future, not be in vain. I believe it can be and shall be done in Esperanto!