The phrase "be to do something" can be replaced with a variety of synonyms depending on the context. For example, "need to do something," "must do something," "have to do something," and "should do something" all convey a sense of obligation or necessity. "Ought to do something" and "be supposed to do something" have a similar meaning, but also imply societal or cultural expectations. "Wish to do something," "want to do something," and "desire to do something" suggest a personal desire or motivation. Finally, "intend to do something" and "plan to do something" indicate a planned future action. Knowing these synonyms can help you express yourself more precisely and effectively in your writing and conversation.