Reminder: о-forms (predicatives) can be used to make complete sentences, e.g.
Хћлодно. It is cold.
Make the future with бџдет:
Бџдет хћлодно. It will be cold.
Make the past with бІло:
БІло хћлодно. It was cold.
See also Sentence Types: predicatives with no Nominative subject..