"The clever weasel usually goes to the forum."
Translation:Mustela callida ad forum ire solet.
It is because solere is used with an infinitive of another verb. It is translated to English as "usually goes" but it really should be understood as something like "is using to go" (this is not correct English, but think of it as "used to go" literally transposed to the present tense). Or think of it as another verb that behaves like "want" ot "like". He likes to go, he wants to go, he solet to go.