There are many synonyms for the phrase "knew for certain", which means that you had complete and absolute knowledge or understanding about something. Some synonyms for this phrase include: "had no doubts about", "was completely sure of", "had no question about", "was convinced of", "was certain about", and "was confident of". These synonyms can be used interchangeably depending on the context and the level of certainty that you want to convey. Whether you are discussing a fact, an experience, or an opinion, using these synonyms can help you convey your level of certainty and confidence to others.