Hello, and thank you for taking the time to answer my questions!!
I have been a fan of the series for a long time now, but there a just a few things that i would appreciate clarification upon. :)
Ill get right to it

1) I understand the concept of the seven planes, and that on the seventh, a spirits true form can be discerned. They cannot hide or change it, right? And according to AoS and GE, these forms have recognisable physical traits ( faquarl has tentacles). Also, a djinn needs to take on a form with wings to be able to fly, or legs to walk, etc. My question is, then, how would a spirit fly, if its seventh plane form cannot be changed, even if it has wings on the first couple. Wouldnt it have to chang them ALL into the same thing to avoid being earth-bound? The solution i have thought of is that they are pure essence on the seventh plane, like gaz/liquid, able to move like their physical forms because its just another aspect of them, but the true answer would be appreciated :) and it still doesnt fix everything :(

2) if this is true, in PG, in the very first fight, the spirits take on multiple forms (invisible1, birds2, fightingforms3-6, true7) How do they manage to fight in so many guises? like without using magical attacks, is someone picked up a rock and chucked it in their fighting form, how could their bird form possibly correspond? It seems like they would have to pick a plane to operate on and fight there, but we know thats impossible because they must exist on all planes, right? I hope im making sense :S

3) in AofS, faquarl says to bart that hes avoiding adressing him on the 7th plane, what exactly was going on there?

I would greatly appreciate you taking time to answer these questions, and i look forward to the next bartimaeus novel!!!
Thanks!
Alex

1)This is just my take, but as far I can understand, ordinary physical objects only exist on the first plane. So, if, you have a spirit that exists on the first plane as a bird, it can fly. However air, (being an ordinary substance) does not exist on the seventh plane, so without needing to make use of Bernoulli's principle and assuming bird form, it would simply move through space on the 7th plane. If however it is a bird on the 6th plane it needs air to fly, and will probably conjure it.
2)Therefore, if it throws a rock in one plane, it will only hit another creature that can operate on that plane. We know it is possible for something to exit on only one plane, because in AoS, Farqual notices that Bartimaeus is not addressing him- or, scientifically not giving rise to sound waves- on the seventh plane. Just like the sound waves, a rock would move through only one plane, in a similar manner.