'Become operative' is a phrase commonly used to describe the moment when something starts to work or become effective. Synonyms for this expression can include 'come into action', 'take effect', 'go into operation', 'become effective', 'activate', and 'start working'. These synonyms are interchangeable and can provide a variety of ways to express the same idea. For example, if you want to describe a machine that has started operating, you could say that it has 'been activated', or that it has 'started working'. Alternatively, if you wanted to describe a new policy being implemented, you could say that it has 'gone into operation', 'become effective' or 'come into action'.