The Powers still had to grant wizardry to the Motherboard's children, even sans-Oath, right? As powerful as Dairine was at that time, she couldn't have forced them to do that if they hadn't wanted to.

I think a Yeerk who hasn't been granted wizardry by the Powers would have the same problem comprehending wizardry as any non-wizard, even when they're made aware of it. I think Nita and/or Dairine show their dad some things (spells? Nita's Manual? Spot?), but to him the written Speech just looks like a very pretty script he can't read. He knows magic exists, but even when he sees it happening right in front of him, his brain still tries to look the other way. If he weren't exposed to it all the time, I think he would forget that it's real, just as a wizard who has their magic revoked forgets it ever existed—except, perhaps, as a fun game they used to play when they were younger and happier.

Having direct access to the brain of a wizard may allow a Yeerk to comprehend magic via their perceptions, but supposing the wizard stops being one (ETA: by voluntarily giving it up, is what I mean), and no longer recognizes that magic is a real thing, I reckon the Yeerk is left high and dry. Even if they still remember it's real (and I doubt they would), they never had the ability to command magic on their own, only via their wizard host. If the former-wizard host can't do it anymore, the non-wizard Yeerk certainly can't.

Overshadowing is a thing, though—because, I think, the Powers will never interfere with a person's free will or the consequences thereof. That's why wizardry is given out so sparingly, because the risk of offering it to just anyone who might then go over to the Lone One is too great.

So yeah, I still think a Yeerk's best bet is to talk the wizard into allowing what the Yeerk wants. Threaten their family, convince them it's for the greater good, maybe just let the wizard talk themself into it with "live to fight another day" reasoning. They'll make it up to everyone when they get free. They promise...

~Neshomeh

I can read numbers. I know that complex mathematics are possible. I can even see on paper how equations are constructed. Ask me to solve a complex equation or construct my own, and I will laugh, 'cause my brain didn't come with the facility to do that. Or, from another angle, I understand the concept of basketball and might even be able to get a ball through a hoop sometimes, from a limited range, but expecting me to play basketball is doomed to fail in uncoordinated misery when I run into another player or trip over my own feet or fumble so spectacularly the ball somehow ends up in my own side's basket. {= )

... The latter angle aligns with what you said about a Yeerk only being able to draw on the knowledge already memorized by the wizard, I think.

Maybe the question to ask is how fine a Yeerk's control of its host really is. If a Yeerk inhabits an artist, for instance, can a Yeerk exercising complete physical control of the artist make them paint something in the artist's style? Or, because the Yeerk itself is not an artist, would exercising complete control over the artist result in something that fails to resemble the artist's style, and is instead in the Yeerk's?

Does the Yeerk rather need to utilize compulsion, and make the artist feel the urge to paint, then sit back and let it happen? It would know the instant the artist conceived the notion of painting "help" and could put a stop to it before it happened, so there'd be little risk involved. But, if the artist resists the compulsion, or follows it by deliberately doing bad art, how much can the Yeerk really do about it?

There's no question that a Yeerk can exercise fine control over a person's language and speech faculties, but is that enough to let them compose poetry?

I actually don't remember any instances of Yeerks attempting to replicate things that require creativity in canon, so I'm really curious!

~Neshomeh

(and I'll say this is both very interesting discussion and a question I don't expect to see myself addressing in my fic, so)

One other thing I thought of (which think I came off of Nesh's point) is that while a wizard-Controller could be rough in terms of knowledge being passed to the Yeerks, there's a solid chance they wouldn't be able to do any wizardry without persuading their host to go with it. As I recall, casting a spell generally requires some intent to do so, so even if you got puppeted through all the words and diagrams and what not, it wouldn't actually do anything unless you wanted the spell could go through.

Or I might be off-base there, I'd have to go digging through books to check for that sort of thing.