"The bull is big."

Translation: Vandis rōvys issa.

[jamesjstark07]

Why not vandis rovi issa ?

[Ctxkh1Me]

When using issa and other forms of sagon "to be", you use the nominative form of the adjective, i.e. the same as you would with the subject. I believe it's specifically the postpositive form of the adjective, which matters for the Solar plural form:

• Vandis rōvys issa "The bull is big"
• Vandissa rōvyzy issi "The bulls are big"

As for the reasoning behind why this form is used, I'm afraid someone better versed in grammar would have to explain it.