Translation:The clever weasel usually goes to the forum.
I translated with "solet" kept as a verb (is accustomed to) that needs an infinitive (ire) to complete its meaning: "The clever weasel is accustomed to going to the forum." (English idiom wants "is accustomed" + "to -ing" ; "is accustomed TO GO" doesn't sound quite natural).
These two are not accepted.
The smart weasel normally goes to the forum.
The clever weasel normally goes to the forum.
This one is.
The smart weasel usually goes to the forum.
I wasn't sure whether it was objecting to "smart" or "normally" so played around. I find "normally" and "usually" borderline indistinguishable though in some contexts I imagine I'd favor one over the other. I didn't report anything as I was focused on understanding what the existing constraints are.
I suppose I should have tried "habitually" as well.
It seems to me that English has many synonyms; I think you're right, that "normally" and "habitually" are very similar to "usually." The standard 'textbook Latin' that I'm aware of likes to give "usually" as a way of translating the verb soleō, solēre + infinitive (otherwise, more literally, "to be accustomed to (doing)").