“If this is a criminal case, how can he be forced to testify against himself? All he has to say is Fifth Amendment.”
Read the first sentence of the article. It clearly states this is a civil investigation: “Former President Donald Trump must answer questions under oath in New York state’s civil investigation into his business practices, a judge ruled Thursday.”
I would simple plead the Fifth if that is possible. I would not answer the questions as they are a trap not a legit attempt to enforce civil law. "I do not remember", "I do not recall" or "I do not know about that," could entrap him if there are emails or text messages or a corrupt former attorney sitting in jail willing to lie to get out. Even if something occurred 10 years ago and Trump really does not remember the details, a email or letter from ten years ago could entrap him.