be doing的用法有哪些?

be to do

通常新闻报道中谈论近期即将发生的事件用be to do
例：Police officers are to visit every home in the area.
表示正式或官方的安排、正式的指令或者给予建议，被动式可以让要求或者规定更加客观。
例：Children are not to be left unsupervised in the museum.
必须是人为可控的未来的事件才可以用be to do
例：The President is to return to Brazil later today. （或...will return...）
The disease will appear again.（因为人类不能控制疾病的爆发，不能用disease is to appear again,）
但如果是谈论过去某人在今后遭遇了什么事情或发生什么情况，不管遭遇的事情情况是否人为可控，都可以用be to do
例：Matthew Flinders sailed past Tasmania in 1770, but it was to be a further 30 years before he landed there.
在if引导的从句中，如果主句的事情是if从句的先决条件，则if从句通常用be to do
例：If Lopez is to win gold at the next Olympics, he needs to work on his fitness. （主句先锻炼，从句才能赢冠军）
If Lopez wins gold at the next Olympics, he has said that he will retire from athletics. （Lopez曾说过，如果他赢了下届冠军，他就会退休，从先赢冠军，主再退休，所以从句不能用be to do）

Be about to do

通常在对话中说到马上、很快就要/不要做或发生人为或非人为的事情，用be about to do
例： We are about to eat. Do you want to join us?
Appearing on TV might make her famous, but it's not about to make her rish.