Board Thread:Lore Discussion/@comment-14027812-20131027182513/@comment-213.143.72.174-20131118001350

Remember that the Dragonborn learns Words of Power by using the defeated dragon's understanding of the learned words - or in the cases of the words learned from the Greybeards, we are specifically told that they "gift [hiim/her] with his knowledge". The Greybeards, contrastively, study the Thu'um for years and decades.

It is my understanding that, as has already been mentioned here, the dragons are the true masters of the Thu'um, and may imbue the words spoken with power as they see fit. Humans however were, according to lore, taught to use the Thu'um as a means to oppose the dragons (Kyne took pity on their suffering, Paarthurnax betrayed Alduin and taught mortals to use the voice, etc etc.) - in essence, the only means they learned to use the dragon language for was as a weapon of war.

Taking all of this in context, it would be safe to assume that humans cannot use the Thu'um until they understand each individual word and the power behind it as a dragon does (which either takes lots uf study, or in the case of the Dragonborn, knowledge stolen from a slain dragon's absorbed soul). This would explain why Arngeir and the dragons we encounter can speak without consequence, yet when all Greybeards speak in unison, the earth shakes under their power, and why the Dragonborn only uses the dragon language as Shouts. It also explains why the Dragonborn must learn specific words of power and then use dragon souls to use them; and why he/she cannot create his/her own shouts from words picked up from listening to the Greybeards or the dragons, especially the prone-to-translation-of-what-he-just-said Paarthurnax. I think it reasonable to presume that any human using the words of power which they truly understand will use them as a shout - and those that do not possess the necessary understanding will have no dramatic effect, like the guards addressing the player as "dovahkiin". Arngeir is the only exception to the rule in this theory, but if he is truly the most powerful of the Greybeards, it makes some sense that he is also capable of speaking as a dragon does, choosing when to imbue his words with power and when not to.

It is of course completely possible that all of this is merely a big oversight on account of Bethesda, but if we do have to come up with an explanation for it to still make sense, I believe the above to be at least plausible, if not probable.