Be To + Infinitive; Be About To + Infinitive

"Be to + Infinitive" - a planned or scheduled event

is used to express future plans or arrangements.

• something is expected or scheduled to happen in the future.

  #She is to meet her friends tomorrow- implies that she has a plan or obligation to meet her friends.

• This structure is often used in formal or official contexts, such as news reports or announcements.

• we only use be to + infinitive to talk about future events that can be controlled by people.

  
  

  #The President is to return to Brazil later today, (or ... will return ...) and
  O The comet will return to our solar system in around 500 years, (not The comet is to return ... the movement of the comet can’t be controlled)
  
  

"Be about to + Infinitive"

is used to express something that is going to happen very soon.

#I am about to leave for work" indicates that the person is ready to leave at that moment.

• This structure is commonly used in everyday conversations #He is about to call you- suggests that the person is just about to make the call.