The title says it all. Basically, I'm a new storyteller looking for general rules to follow when dealing with ghosts, demons, etc. The ArM5 book reads like Greek to me (I have a feeling it was written assuming the audience had some knowledge of the previous editions), so I didn't learn much from the material presented in the core book.
If a Magi wanted to affect a ghost with magic, which Form(s) would/could you use? Me, Co, Vi? I take it that ghosts are considered to be of the 'magical' realm, so is it safe to say that ghosts are not souls, but some sort of different 'magical' manifestation of a dead mortal? This is all still very fuzzy to me, so any clarification on how different forms affect different creatures would be much appreciated.
Hermetic magic cannot affect the Soul. Fortunately for your mage, ghosts (as you rightly point out) are not souls, but spirits.
Ghosts can be affected by generic vim, since vim affects all supernatural creatures. They can also be affected b y Mentem. Generally Mentem might be more useful, since the difficulty level of the spells will be lower than using a generic Vim spell designed to affect any supernatural creature of the magic realm
I don't have my books with me, (so please correct me if I"m wrong) --
but I believe you'll find a lot more information regarding ghosts in the following books:
Houses of Hermes: True Lineages (Tremere Section, with lots of good spells on communicating with and controlling ghosts)
Mysteries: Revised Edition (discusses Magi Immortality via becoming a "living ghost")
I'd also suggest searching this forum. Ghosts are pretty common topic.
Just to clarify a few secondary-points you referred to:
Ghosts can be of any Realm of Power. Some ghosts are spirits of the Magic Realm, and these are probably "typical" ghosts. There are also Infernal ghosts, and Faerie ones, and possibly even Divine ones.
I believe ghosts are said to be the spirit of the person, as opposed to his soul or body. Spirits are things without bodies, they are purely mental beings and hence affected by Mentem. This doesn't mean that just their thoughts are subject to Mentem - they are Mentem creatures, and as such it is as if their "bodies" are made of Mentem. Rego Mentem can ward off ghosts, Muto Mentem can make them visible, Perdo Mentem can harm them, and so on. As supernatural beings, they are also affected by Vim, as noted above.
Demons (and angels) are not ghosts. However, they thoughts can still be affected by Mentem since they have a mind (in game terms - Intelligence), and they can still be affected by Vim since they are supernatural beings. When donning a corporeal body, they can also be affected by the relevant Form (often Corpus, sometimes Animal or Ignem or whatever). If you're considering handling either demons or angels, there is more to be said on each. Most importantly, both are actually souls and can abandon corporeal bodies or create new ones as needed; and Hermetic Magic is unable to pierce the deceptions of demons.
If demons/angels/daimons can have their thoughts affected by Mentem, what about the thoughts of magically intelligent animals? Are they Animal or Mentem? How does "Opening the Tome of the Animal's Mind" work on them?
My understanding is that if it has Intelligence - and all demons/angels/daimons do - then its thoughts are affected by Mentem. Which doesn't mean it's easy to target the critter, or that special rules don't apply (you can't detect demon's lies, and you can't force an angel to act in an unholy manner).
If a magical animal has Cunning, it is affected by Animal guidelines as normal. If it has Intelligence, its mind is affected by Mentem. In this case OtTotAM won't work on it; at least not without a Mentem requisite.
That's incredibly relieving to me actually. I don't suppose there's a page reference in either the main book or any splatbook to support it? Possibly RoP: Magic (which I don't have yet, but am considering)?
The answers above are good, but I'd like to clarify a bit more.
Supernatural beings can be warded or destroyed in two basic ways.
Target their Form - in the case of a Ghost, Mentem or Corpus depending on whether the ghost is Corporeal or not. This approach is probably the better choice for a necromancy specialist since he can target Divine, Faerie, Infernal or Magic ghosts with a single spell.
Vim - Since all ghosts have Might and are associated with one of the four supernatural Realms, they can also be affected with Vim though since each spell mut be tailored to a specific Realm, this is less effectual for necromancers. Can be advantageous if one wants to ward all Infernal spirits, however, since it differentiates targets solely on the basis of their realm affiliation.
According to HoH: TL, ghosts are sort of a spiritual echo left behind by the human soul before departing its body.
In this same vein how do you all see a spell like Posing the Silent Question working with a ghost? Let's assume a spirit tied to the magic realm. If the spell penetrates would it function the same way it would to a human mind? I.e. the magus needs eye contact (what if the ghost no longer has eyes) and the spell will answer a question as accurately as the spirit's mind allows.