And I would argue that you do not. You need to have been told: "When I start drawing on the ground, don't step in it!" - which is hardly an XP in anything.
Recognizing it as "Yeah, probably magic" requires... again, probably very little, but can be done with magic lore as well as magic theory.
Recognizing it as "Probably a Ward, yeah?" well, most traditions use Circular Wards, because they are practical. Again: Magic Lore or Magic Theory.
Recognizing it as "Yup, Circular Ward Against Demons, probably 3rd magnitude" - Magic Theory for sure.
However, going back to the first post:
Vis and Realms - Realm Lores.
And Wards are ubiquitous enough in magical traditions that I'd let this be covered by Magic Lore - or indeed any Realm lore, since all four Realms use them.
I'm not saying that there's no point in teaching grogs Magic Theory - but for the sort of 'general knowledge' about the basics of magic that many seem to refer to in this thread, I'd go with Magic Lore rather than small amounts of magic theory.
It's much like teaching kids that "you shouldn't go in front of a speeding car, if you get hit you'll be hurt and maybe even die" instead of starting out by teaching them about conservation of energy and momentum, and then using that to explain why stepping in front of a speeding car is a bad idea.
EDIT: Editted to include (hopefully all of) the words I forgot to type the first time around.
Knowing magic theory is OK. A grog can learn it through teaching, and it makes him useful in some specialized areas. Since it is NOT necessary that the teaching is performed by a magus, I see no problem with this. I assume that quite a few grogs can develop it even by exposure, by looking at the magi do stuff and interacting with items.
Now, we have always found that knowing Legend Lore (our version of Fae + Magic lore, since we only play with 2-3 realms; there is hermetic debate on the separation of infernal and divine) is much more useful. General magical stuff is not MT but Magic Lore. MT deals with hermetic magic alone, not with magic in general. Regios and aura interaction is not magic theory for sure. Not IMS, at least. For hermetic spies that gather info on the arts strengths and weaknesses of opponents it is invaluable, though. Most of our grogs have a score in Legend Lore, even if it tends to be low, it helps them to not freak out at every little thing they will encounter.
That might leave your shield Grog out of the ward you are just tracing to protect both of you, of course.
Anyway this looks like an issue of covenant culture to me - and covenants treating Grogs like you suggest certainly do exist. As do SGs exploiting that.
This may be a problem with with my imperfect understanding/use of english. When I state "When I start drawing on the ground, don't step in it!" I seek to convey the intent of "Please do not step on/smudge the lines - even the unconnected wiggly ones - that I'm drawing" not "do not under any circumstances step inside the circle I have drawn."
I thought the fixed expression for this was stepping "in" it? Would "When I start drawing on the ground, don't step on it!" have been more correct?
It is rather the problem, that with "When I start drawing on the ground, don't step in it!" you don't tell your shield Grog just where he has to stand to defend you while you are drawing the ward, and just when he has to go where to benefit from the ward coming up.
And no, while you are drawing and he is defending it against some creature, you just cannot explain that all to him, no matter how precise your English might be. He needs to decide himself how to shift and weave to keep you and your diagram covered, and he also needs to decide when and how to disengage and fall back into the ward, without destroying it.
...and my argument is simply that this knowledge is contained within Magic Lore - which is likely to be more widely useful to a fighting grog - as much as within Magic Theory, but in a much more useful format.
Failed apprentices don't have the Gift, but are often of assistance in the lab, although I recognize that's a function of the Virtue, since it is stated explicitly there. But, I would go so far as to extend the Servant section of Covenants in the Lab Personalization rules and have the bonus equal to .5(Int+MT).
I already quoted in this thread the very short definition of Magic Lore (ArM5 p.66), and really can't see how timing, procedure and conditions of casting an Hermetic ward would be covered by it.
That is because they are not. A hermetic ward is hermetic magic and hence covered by MT. However I would argue that passing familiarity (the magus whacking your head repeatedly) is enough to earn that small part of MT knowledge without having to invest XP there. Magic Lore covers regios, recognizing general spellcasting (both hermetic and not hermetic: when you see a guy giggling and muttering werid stuff), identifying sources of power and auras, identifying magic creatures and all that crap that can generally make the life of a grog quite miserable. Apart form the mundane stuff (and infernal, and divine, and...) that also makes his life miserable, that is.
Yes, of course a grog doesn't need Magic Theory to do whatever a magus tells him to do (or not do) around wards. He just needs to be obedient and have the capability to understand the instructions (although, of course, some grogs sadly lack these characteristics).
However, a grog with Magic Theory has some chance of knowing what to do when some circumstance arises that a magus hasn't provided instruction for. For example, a grog with Magic Theory has a chance of knowing whether a "ward" he stumbles upon is an Hermetic Ward, or a non-Hermetic Ward, or merely a chalk circle on the ground. And he has a chance of identifying what a (Hermetic) ward is actually warding against, if he sees it being cast. And he has a chance of understanding (without being told) precisely what does and does not constitute "breaking the ward".
He maybe doesn't have a very good chance of knowing this stuff (if he has a low Magic Theory Score and/or low Int, for example). But he has some chance if he has Magic Theory.
All of this is quite handy for grogs to know when they are out Adventuring, and many grogs could acquire such knowledge eventually through Exposure. Of course, it may not be handy enough for a magus to consider "wasting" a season teaching it. But, grogs don't need to be taught Magic Theory by a magus. A literate grog can read a book on it, and illiterate grogs can be taught Magic Theory by other grogs.
A custos with magic theory 3+ can set up and potentially even refine a lab. This can be handy if your lab gets trashed and you want to spend a couple of seasons reading a book rather than sorting out a new lab.
For combat grogs, a knowledge of magic theory means they'll recognise spells. They know what Perdo Corpus looks like, and can distinguish it from Intellego Corpus. This is important; they know that the hostile magus casting InCo is probably up to no good but isn't going to be a problem right now, while the one casting Perdo Corpus wants a slap upside the head to break his concentration.
Actually, knowing that you can break concentration is also very, very useful information!
The moment you start equipping your grogs with a few magic items designed to interrupt/interdict enemy spellcasters the need for their having some MT goes up. Just make darn sure they are loyal, or you may find yourself being the one interdicted.
A shield grog covered by Parma and with the right magic items can be nearly as dangerous on the battlefield as a magus. They can know/guess the kinds of wards a magus is likely to have up, and work around that. It does not take many items at all to make a grog into a magus-killer, and by and large Hermetic magi are probably one of the most dangerous things your troupe will face.
All you need to do is envision just what you can do with a teleporting grog with a range of tools designed to slip past wards and acquire blood arcane connections. Add in that grog being able to spot what you're casting, and you've got a seriously dangerous fight on your hands.