If you wanted to say in Esperanto something like:

"In three years, I will have been living here for a decade."

or

"By tomorrow, I will have lost 20 pounds since I started exercising."

I'm getting tripped up on instances where you might say "will have" in English, referring to actions that started in the past but will be completed in the future. Would you just use the future tense for this?

It's the future perfect, which is a periphrastic construction in this lang. The first example is:

Post tri jaroj, mi estos loĝinta ĉi tie ekde jardeko (or "por jardeko").

As Roberto said, this is one of the (few) cases where you must you a complex verb form. Here we describing a future state of an action that has been in progress for awhile.

In "In three years, I will have been living here for a decade" the easy parts are "in three years", post tri jaroj, and "here", ĉi tie.

After those three years the action of living the state "comes to being", "emerges", estos, and at that point the action is still in progress, thus loĝanta. The form loĝinta would mean in my book that the living would have been ended.

To me the last piece, "for a decade" is a tricky one. My understanding is that it may not be translated to por jardeko because the general meaning of the preposition por is goal or purpose. As an extension of this por is used with time to denote a length of a future time which the action must last. For instance forvojaĝi por tri semajnoj, to travel away for three weeks. My reading is that here we do not have any must, rather a simple expression of a period. That is expressed by tempa akuzativo, so jardekon.

Putting all pieces together:
 

• Post tri jaroj mi estos loĝanta jardekon ĉi tie.

PS. I think that many would opt for a simpler structure at least when speaking: Post tri jaroj pasos jardeko ekde tiam, kiam mi ekloĝis ĉi tie.

Roberto12:It's the future perfect, which is a periphrastic construction in this lang. The first example is:

Post tri jaroj, mi estos loĝinta ĉi tie ekde jardeko (or "por jardeko").

Thanks.

Metsis:As Roberto said, this is one of the (few) cases where you must you a complex verb form. Here we describing a future state of an action that has been in progress for awhile.

In "In three years, I will have been living here for a decade" the easy parts are "in three years", post tri jaroj, and "here", ĉi tie.

After those three years the action of living the state "comes to being", "emerges", estos, and at that point the action is still in progress, thus loĝanta. The form loĝinta would mean in my book that the living would have been ended.

To me the last piece, "for a decade" is a tricky one. My understanding is that it may not be translated to por jardeko because the general meaning of the preposition por is goal or purpose. As an extension of this por is used with time to denote a length of a future time which the action must last. For instance forvojaĝi por tri semajnoj, to travel away for three weeks. My reading is that here we do not have any must, rather a simple expression of a period. That is expressed by tempa akuzativo, so jardekon.

Putting all pieces together:
 

• Post tri jaroj mi estos loĝanta jardekon ĉi tie.

PS. I think that many would opt for a simpler structure at least when speaking: Post tri jaroj pasos jardeko ekde tiam, kiam mi ekloĝis ĉi tie.

Thanks for the explanation. I probably need to understand participles better.

What if you were saying something like, "By tomorrow, I will have finished writing my letter." How would you translate that?

It's odd to me that a phrase like "for a decade" would be tricky. That seems like such a simple and common type of expression that I would think Zemenhof and Co. would've quickly developed a way to say this sort of thing.

"By tomorrow, I will have finished writing my letter."

In the future (estos) the action (skribi) is in a state of being ended. So
 

• Morgaŭ mi estos skribinta mian leteron.

But since the idea is to emphasise finishing of the writing, a better verb might be kompletigi, which gives us
 

• Morgaŭ mi estos kompletiginta mian leteron.

which can be simplified to
 

• Morgaŭ mi kompletigos mian leteron.

I have noticed that in many cases you need to think how long an action lasts. While writing is a long lasting action, finishing is a short one and that can be expressed with a simple verb.

Maybe it is just me but I find sometimes time expressions in Esperanto ,well, odd might be a too short word but shall we say, requiring thinking.

We have antaŭ and post to express that something happened, happens or will happen before resp. after a certain time reference. There is no connotation to length in antaŭ and post, so they are points in time. Then we have ĝis and de which work in the similar way except they have length.

antaŭ with past, present and future event

• Ni manĝis, antaŭ ol ri alvenis
  
• Antaŭ kelkaj jaroj mi vizitis Seulon
  
• Antaŭ la sekva semajno mi ne povas partopreni

post with past, present and future event

• Post kelkaj jaroj da malliberiĝo oni amnestiis rin
  
• Kiel ne venis al vi en la kapon diri, ke post monato oni povas sciiĝi ankoraŭ pli bone
  
• Mi forveturos post monato de hodiaŭ

ĝis with past, present and future event

• Mi konsideris ĉiujn proponojn, kiujn estis senditaj al mi ĝis tiu dato
  
• La rezultoj, kiuj sin montris ĝis hodiaŭ, plifortigas mian kredon
  
• Mi konsilas atendi, ĝis la komitato donos sian decidon

de with past, present and future event

• Jam de monato ili ne ricevis sian salajro
  
• Kia bedaŭro ĝi estos por tiuj, kiuj atendadas de monatoj por ekvidi la spektaĵon
  
• Komencante de morgaŭ…

Note that while in Esperanto you have only four prepositions, I need ten different expressions in my native language to translate those all (and I doubt whether four is enough in English too). There is definitely no one-to-one mapping.

In "In three years, I will have been living here for a decade" we have post + future event. Now this future event has its own time description. It is neither a duration (dum), a single event (en) nor a subject (la pasinta jardeko estis…), but something (yet more) different. For me tempa akuzativo is piece of cake since my native language has it, but I can well imagine that it is not for all.

Metsis:"By tomorrow, I will have finished writing my letter."

In the future (estos) the action (skribi) is in a state of being ended. So
 

• Morgaŭ mi estos skribinta mian leteron.

But since the idea is to emphasise finishing of the writing, a better verb might be kompletigi, which gives us
 

• Morgaŭ mi estos kompletiginta mian leteron.

which can be simplified to
 

• Morgaŭ mi kompletigos mian leteron.

I have noticed that in many cases you need to think how long an action lasts. While writing is a long lasting action, finishing is a short one and that can be expressed with a simple verb.

Thanks for the explanation.

I'm going to have to think about this and try to get my head around it, because the phrasing here does not naturally seem to mean what it apparently means. For instance, if I see "Morgaŭ mi estos skribinta mian leteron," I naturally read it as, "Tomorrow I will be written my letter," which just comes off as the kind of gibberish someone might say if English is their second language.

My knowledge and understanding of participles is woefully deficient. I should probably take a week to really study that subject and try to get it down.

Metsis:Maybe it is just me but I find sometimes time expressions in Esperanto ,well, odd might be a too short word but shall we say, requiring thinking.

Thanks again for the detailed explanation.

Some of this also is challenging for me because of how unnatural it sounds in my head. For instance, "antaŭ kelkaj jaroj" . . . "before some years," when translated literally. I think that, like I was saying with participles in my last response, learning these time expressions is something that might require its own dedicated specialized study.

I'm going to look over everything you posted in more detail as I have time to see what sense I can make it of it.

After those three years the action of living the state "comes to being", "emerges", estos, and at that point the action is still in progress, thus loĝanta. The form loĝinta would mean in my book that the living would have been ended.

Really, Metsis? I haven't checked , but is there any support from the Tekstaro for your preference for 'loĝanta'.

In my book estos loĝinta doesn't imply the intention to move out.

If you want to avoid the complex form you could say Post 3 jaro mi loĝos ĉi tie jam dek jarojn. But I find 'estos loĝinta ĉi tie jam dek jarojn' more elegant.