The construction "to be + infinitive" has a future meaning but its use is more limited than "will + verb". It is used when a decision or plan has been made and consequential action will soon follow. It is a formal construction and often used when reporting events e.g. "The government is to invest 20 million euros in the construction of a bridge connecting Town A and Town B." In this case, the government had already decided on the investment, and it will be made soon.

They are very similar; often we could use them interchangeably- "I am to get married next year!" "I will get married next year!" However, "I am to do..." feels a little old-fashioned and isn't as commonly used in my experience. It also could suggest an obligation- what you are SUPPOSED to do or what has been planned for you- while "I will do..." suggests more what you INTEND to do or what you plan for yourself. The difference is very subtle. A: What part did you get in the play? B: I am to be the king's servant. A: Oh, that's not what you wanted. Do you still want to be in the production? B: I'm not sure. I will think about it. When in doubt, use "I will do." It's a little more common. Hope that helps!

English is a messed-up language. Grammar is not simple. "To" is not part of am. "To" belongs with "do". "To do" is the infinitive form of "do". But "am" sounds very strange to me. Ex: I have to do something. I have something to do. ->You don't "am" something. ->You "have" something. ->"Am" doesn't make sense in this situation. (I am to do something. I am something to do.) MAYBE YOU CAN ALSO UNDERSTAND BY THE WAY : IAM TO DO = I AM GOING TO ( BUT THIS IS JUST USE WHILE SAYING ) Good Luck !

You may use then in different situations. The latter is more optional than the former,when you use the second explanation,you dont have to add up certain places or time to make it accurate,unlike the first one.

be to + do = should/could/might/would/shall/can/may/will + do(具体是哪个情态动词要根据上下文判断)