One day the man could not but go to a market.
The sentence is to be transformed into affirmative. But I am just not getting how to transform it using 'must'. If I write- 'The man must go to a market', then it makes a sense of present tense...but it has to be in past form as the negative sentence was in past form. So I thought about this- "One day it was must for the man to go to a market". I'm not finding any other way except it.