"That food is pretty strange."
Translation:Tuo ruoka on melko outoa.
The subject, and any adjective preceding it in the attributive position, is most often nominative. After the verb on in the predicative position, the case of the adjective depends on the subject.
If the subject is a singular countable noun, like koira, then the predicative adjective is nominative. Thus Tuo koira on outo.
If the subject is a singular uncountable noun, like ruoka or vesi, then the predicative adjective is partitive. Thus Tuo ruoka on outoa.