If someone has a pussiant ability (like and Bonisagus magus), and they teach this ability, what is the study limit of their student?

Can they teach to the level indicated by their unmodified ability, or to the modified ability?

As the book states, it would appear that two Bonisagus magi can teach each other MT for all eternity!

Same effect applies to books... Can they write summa to their adjusted level? And can they write 1 extra tracus (compared to others with their unadjusted level)?

'Puissant Ability' has been errataed in http://www.atlas-games.com/arm5/arm5errata.php:
//Puissant (Ability) (p. 48): Replace the first sentence of the text with: "You are particularly adept with one Ability, and add 2 to its value whenever you use it. Note that you do not, in general, use an Ability when learning it, teaching it, or writing about it."//

From this erratum your questions can be answered:
//Can they teach to the level indicated by their unmodified ability, or to the modified ability?//
//Can they write summa to their adjusted level?//
//And can they write 1 extra tracus (compared to others with their unadjusted level)?//
It is 'no' on all three counts.

Kind regards,

Berengar

Ahh - misinterprited the phrase "use the ability"...

Thanks:)