Mr Mind's page

In the Core Rule Book FAQ I cannot find anything on Use Magic Device.

So, RAW says, "Emulate a Class Feature: Sometimes you need to use a class feature to activate a magic item. In this case, your effective level in the emulated class equals your Use Magic Device check result minus 20. This skill does not let you actually use the class feature of another class. It just lets you activate items as if you had that class feature. If the class whose feature you are emulating has an alignment requirement, you must meet it, either honestly or by emulating an appropriate alignment with a separate Use Magic Device check (see above)."

UMD would not allow you to use Wild shape by itself. However, it would allow you to activate a magic item that produces a Wild Shape effect. If a fighter has use magic device and wears a Druid's Vestment then she could activate it to emulate the Druid's Wild Shape ability and activate the vestment's magic 1 time each day (if she can make a UMD check of 24 or higher). The UMD would allow her to emulate a class ability to then activate the magic device.

Assuming she has an INT or CHA of 13 or more, if she picks up a staff of fire and makes a UMD check of 32 then she could cast a fireball at caster level 12 (it still only does 10D6 damage but she would roll as caster level 12 to overcome spell resistance).

If she picks up a page of spell knowledge (level 1) then she could roll a DC 21 UMD to know the spell. However, she still cannot cast the spell since she doesn't actually have any spell casting ability. She could likewise roll UMD to use a ring of wizardry I. However, she would not gain any extra spells since it "doubles" the character's 1st level spells (zero doubled is still zero). But, if some magic item allowed a sorcerer to cast magic missile 1 extra time per day as long as she knew the spell then, the fighter could use a UMD to activate a page of spell knowledge (magic missile) and then another UMD to activate the hypothetical magic item to cast magic missile (at caster level UMD check -20).

The rules are relatively straight forward. I'm not sure what the confusion is.

I recently had this come up and this is my RAW handling of it. If you look at each class entry, Cantrips is a class feature separate from Spells. Therefore, any Cantrip (or Orison for the same discussion on Orisons) that is listed in a legally available PFS publication can be prepared in a Cantrip slot. However, since Cantrips are a separate class feature form spells, then Spells cannot be prepared in Cantrip slots. Luckily, since the rules state that Cantrips, "are cast like any other spells" then meta-magic feats can be applied to Cantrips. In that case they take up your # per Day slots as usual and would then be limited in how many time you can cast them.

So, in summary, Cantrips and Orisons are separate class features from Spells and, given their unique ability to be continuously recast, cannot accommodate Spells (in the rules mechanic - class feature, RAW sense of the word).

So, a Merciful Ray of Frost or Acid Splash can be cast all day long. However, a Merciful Magic Missile or Shocking Grasp, although mathematically a 0th level spell, must still take up the MINIMUM available non-cantrip slot which is still a 1st level spell slot.

"Cantrips: Wizards can prepare a number of cantrips, or 0-level spells, each day, as noted on Table 3–16 under “Spells per Day.” These spells are cast like any other spell, but they are not expended when cast and may be used again. A wizard can prepare a cantrip from a prohibited school, but
it uses up two of his available slots (see below)." - Core Rule Book page 79

"Spells: A wizard casts arcane spells drawn from the sorcerer/wizard spell list presented in Chapter 10. A wizard must choose and prepare his spells ahead of time." - Core Rule Book Page 78