Luna will not destroy thou · 9:57pm Aug 10th, 2013
Mostly because that's not the right way to use "thou".
Quick history lesson before I get to the practical gist of this.
A lot of languages have two different second-person pronouns for various degrees of formality. In Spanish, "usted" is used to address strangers and superiors, whereas "tu" is used to talk to friends and equals. At one point, English had something similar: "you" was reserved for addressing superiors, whereas "thou" was used for pretty much everyone else. Eventually, egalitarianism kicked "thou" to the curb, and "you" became our only second person pronoun (not counting "y'all").
The upshot of this is that thou is weird, and not to be trifled with. But the real problems come when we bring "thee" into play.
It's really not complicated, but because of thou's relative rarity, it's frequent that people don't take the time to learn. In 99.9% of things written, that's not a problem. However, when you're writing Luna's dialogue, you have to be careful.
This is because there's a difference between "thou" and "thee". It's basically the same situation as "I" and "me". When the subject of your sentence is the "second person," you use "thou", and when it's not, the correct pronoun to use is "thee".
Here's an example.
Thou hath failed to use these pronouns correctly. For this insolence, they shall destroy thee.
This is actually the subject of a blog post I made a couple weeks ago:
The Royal Canterlot Voice and You: A Quick and Dirty Guide to Speaking Far Too Loudly
Things like this seem very simple to those of us with some romance language experience, but... English is weird. I didn't learn hardly any grammar until I started studying French and Spanish. I like to tell people I learned English in French class.
I like instructive blogs like these. You just earned yourself a stalker!