The suffix -von was invented to help indicate progressive tense as an equivalent suffix to English's -ing ("bovon" = "flying", "kriivon" = "slaying", etc.). It originated from the canon word "Sindugahvon", listed to mean "unyielding".

I have been making a new batch of lessons and I decided to look into if our reverse-derived word "Gah" really means "to yield", and as a result if -von really works as -ing. UESP lists "Gahvon" as "to yield", citing in-game dialogue. I dug into the dialogue files myself and came across this line of Odahviing's:

Dovahkiin zol mul. Zu'u gahvon miri Alduin.    Dovahkiin is most strong (=stronger). I yield my allegiance (to) Alduin.

The italicized part is my own emphasis and shows the actual line of dialogue. The other part is notes on the translation by Bethesda.

I think this is fairly conclusive evidence that the proper word for "to yield" is "Gahvon", and that -von is incorrect as a progressive tense suffix. There are three main questions I see before us with this new information:

1. Do we terminate the suffix -von?
2. If so, do replace the suffix?
3. If so, what should the new suffix be?

1. Do we terminate the suffix -von?

The suffix is featured fairly heavily in existing writing and terminating the suffix would present a great challenge in being able to read and translate those writings. I think we should terminate the suffix because it is incorrect. Then it becomes an issue of how to make those writings still accessible to someone who is either unaware that the suffix has been terminated or unaware that the suffix ever existed.

2. If so, do we replace the suffix?

I'll be honest, I have never really liked progressive tense in Dovahzul. The lack of (most) tense is what makes Dovahzul unique, and I always felt -von was overused. An argument for not replacing the suffix might be to enforce that no-tense philosophy. Of course, we can't ignore something that already is in canon, which is very good news for people who want a progressive suffix and are wondering about the answer to the third question.

3. If so, what should the new suffix be?

There is one other canon example of a word that is explicitly given a definition that contains "-ing": "Viintaas", "shining". What's great is that canon also explicitly defines the root word "Viin", "to shine". From this we can very decisively conclude that -taas may be a progressive suffix of a sort. Our previous examples then become "botaas", "flying", and "kriitaas", "killing".

If we make -taas our new progressive suffix, what is the best way to ensure everyone knows about it and keep older writing decipherable?

According to your dictionnary, "sindugahvon" is the only word beginning with "sindu", and apparently, you're saying that both "gah" and "gahvon" mean "yield". What if the prefix "sindu-" is used to create a negative progressive tense? An equivalent to "un(insert.verb.here)ing", "sindudreh" meaning then "undoing". Then, as you proposed, we could also use the suffix "-taas" to create affirmative progressive tense.
I agree it would be a bit harder to assimilate this language, but thus, it would respect the translations of "sindugahvon"="unyielding", "gahvon"="yield" and "viintaas"="shining".

"Sindu-" for "un(verb)ing" and "-taas" for "(verb)ing".

About keeping the old rule still decipherable, we could add the rule that "-von" isn't a suffix if e have the prefix "sindu-", otherwise "-von" is a synonymous os "-taas" with "gahvon" as an exeption.

"Vo-" is already a well-established prefix for "un-". Not that we can't have two, but many words make use of "vo-" while "sindugahvon" is the only one that makes use of "sindu-", so it is more an exception than a rule.

"Gah" was previously entered in the dictionary to mean "yield", but this is incorrect. "Gahvon" means "yield". This means "von" isn't a suffix in any canon sense. Although it would be against my preference, we could continue to have "-von" as a lesser used, synonymous suffix to "-taas".

paarthurnax

"Vo-" is already a well-established prefix for "un-". Not that we can't have two, but many words make use of "vo-" while "sindugahvon" is the only one that makes use of "sindu-", so it is more an exception than a rule.

I might have not explained clearly my thoughts about my hypothesis: "Sindu-" would replace the use of "Vo-" and "-taas" when they are in the same word. For example, with the word "Dreh" which means "to do", we would have "Vodreh" = "undo", "Drehtaas" = "doing" and "Sindudreh" = "undoing". A 2-in-1 suffix, creating a new tense, the negative progressive, while "Vo-" indicates the English prefix "un-" and "-taas" marks the affirmative progressive tense.

I understand it wouldn't be easy to apply this new rule, but I think this is the easier one if we want to avoid exeptions which make a language diffucult to learn: if French is so hard to learn, this is because of exeptions and exceptions os exceptions (it's even hard for native French speakers).

paarthurnax

"Gah" was previously entered in the dictionary to mean "yield", but this is incorrect. "Gahvon" means "yield". This means "von" isn't a suffix in any canon sense. Although it would be against my preference, we could continue to have "-von" as a lesser used, synonymous suffix to "-taas".

Well then, in my opinion, there should be a little correction for "Gah". And I agree with you regarding "-von" and "-taas".

Oh! I understand now, that's a very interesting idea. It wouldn't be a problem to add "sindu-" as a prefix. Finding an elegant way to remove "-von" is a problem apart from simply saying "don't use '-von' anymore", which just might be what ends up happening.

I've been inspired from some Japanese rules, and I tried to think about the addition of "sindu" to "gahvon", so, I came up with this idea.

About the transition from "-von" to these rules, maybe the lessons couold be modified like this in the suffix table:
________________________________________________________________
|-von (archaic)     | equivalent of "-ing" (affirmative and             |                    |
|                            | negative, not in use anymore                         | (examples) |
|                            | old form of "-taas"                                           |                    |
|                            | doesn't work with "Gahvon"                          |                     |
|______________|_____________________________________|___________|
|-taas                    | equivalent of affirmative "-ing"                       | (examples) |
|                            | new form of "-von"                                           |                    |
|______________|______________________________________|__________|

And adding "Sindu-" as being a prefix replacing the addition of "vo-" and "-von" in the same word.

I thought about something:

Maybe, calling the sufix "taas" a 'positive progressive' and the prefix "sindu" a 'negative progressive' isn't the best way to refer to these affixes. I think we would reather have to call "taas" a "progressie" and "sindu" a "degressive". It's actually just playing on the words, but I think it would be easier to understand their function if they're called like this instead of affirmative and negative progressive Moreover, calling "sindu" a negative progressive could be quite confusing and people could believe it's the equivalent of "don't V-ing" instead of "un-V-ing".

Drem yol lok Paarthurnax! I was worming around your twitter/blog/thingy, and I watched a videoa from an older post http://www.youtube.com/watch?feature=player_embedded&v=a5mZ0R3h8m0, and that brought me to some interesting ideas. I am not a language expert, and honestly I have know clue what you and Drakpa are even talking about, but here is my take on things. First of all, this is our language to construct, and is totally original. There is no way that it is going to be perfect, and like any other language, there are bound to be exception. Gahvon could just be one of those exceptions. Also, remember what I said, we can make anything we want up (as long as its not ridicously clumsy, stupid, and doesn't make sense). We could have two versions of the prefix for -ing. Basically, TES dovahzul is just a template to launch this language into something new, the only thing we need to be careful is if Bethesda adds any more words/rules into the mix, and that might be this community's uprising or downfall. Overall, I believe -von should stay.

As for the rule of how the dovahzul version of -ing works, is entirely up to you two and anyone else who joins this thread