Can someone provide some clarification on the impact of Forms on Magic Resistance. In ArM4 I always assumed you added your entire Form score to Magic Resistance, but I thought that changed in 5th Edition. Under the Hermetic Arts section, both books state:

"In reflection of this, your socre in a Form (in addition to allowing you to cast spells incorporating that Form) helps you resist spells of that type and avoid harm from mundane sources related to that Form. For instance, your Ignem (fire) score gives you a bonus to resist damage from fire and cold."

Only 5th Edition goes on to say: "This bonus is equal to one fifth of your score in the Form, rounded up, and adds to your ability to resist any damage deriving from that Form."

I assumed this was a modification to the mechanics, in light of the new Penetration rules, thus making the Forms have a lesser impact overall on magic resistance than the Parma Magica. However, under the Magic Resistance section of 5th Edition, it reads:

"Hermetic magi have a base Magic Resistance equal to their score in the Form most applicable to the spell cast. <snip> Thus, a maga attacked with magical fire would have Magic Resistance at least equal to her Ignem score."

Two questions.

First, if a magus has a Parma Magica score of 3 and an Ignem score of 10, which scenario would apply (in regards to a fire-related spell):

1) The magus has a magic resistance of 25 (15 from Parma + 10 from Igem).

2) The magus has a magic resistance of 17 (15 from Parma + 2 from Ignem (10/5).

Second, would the bonus to resist damage only apply to mundane damage sources, or would it also apply to the magical damage sustained from the effect of a spell, if the magic resistance failed to protect the magus from the initial spellcasting?

"Two questions.

First, if a magus has a Parma Magica score of 3 and an Ignem score of 10, which scenario would apply (in regards to a fire-related spell):

1) The magus has a magic resistance of 25 (15 from Parma + 10 from Igem).

2) The magus has a magic resistance of 17 (15 from Parma + 2 from Ignem (10/5)."

It should be the first - add the full form score to your parma. This is still like 4th edition in that aspect. Form score + (5 x Parma Score) for your resistance.

"Second, would the bonus to resist damage only apply to mundane damage sources, or would it also apply to the magical damage sustained from the effect of a spell, if the magic resistance failed to protect the magus from the initial spellcasting?"

Since it talked about Form bonus resisting any damage from that source, I would conclude that this would also apply to soak against magical fire. You would get the form bonus (in your example above, +2) to add to your soaking the damage.

that's right: you add the full Form score to your Magic Resistance;
you add (Form/5) rounded up to your Soak and certain other defensive rolls (e.g. Aq: drowning & thirst). Each Form lists examples of the areas in which the Soak bonus applies.

Note on Form & Parma:
* you can share your Parma with others,but they don't get or share your Form bonus (though an Apprentice might have their own Form bonus!)
* you can suppress your Parma temporarily, and this also suppresses your Form bonus (p.85, col 1); this is a conscious act equivalent to Concentrating on a spell - and can't be done while unconscious.
* an unconscious maga will lose her Parma at sunrise/set, but cannot suppress her Form based resistance.