是不是主语加be动词加短暂性动词，表示将要做某事？
您所问的是be + V-ing形式的“现在进行时”结构句型的用法吧？
C1: 如果说话的当时，正在发生，用现在时行时；
例句：
It's raining outside. It's raining right now (but it may stop soon).
Sarah's working in the library. She's working there right now.
Tom is spending Christmas with his family. He's spending Christmas with his family right now, this year. (Maybe next year he won't.)
C2: 如果是谈论近期已经确定的计划和安排，也是用现在时行时；
I'm playing soccer tomorrow. This plan is already arranged and definite.
Sarah's leaving for San Francisco on Friday. She has probably already bought her ticket.
The Olympics are taking place here next year. This is already certain.
I'm having a party next week. All the plans have been made.
形式上都是“现在进行时”，但未必说的是现在正在发生的，还可以是近期的计划（如playing, having 就不是短暂性动词），或者马上要发生的事（leaving, coming, dying, getting married... 这些是短暂性的动词，显然不会是正在进行，而是表示“快要发生”，即：快要走了，快要来了，快死了，很快要结婚了）。
学语法，不能只记形式和规则。只要是有这种用法，就是合理的。
功能一，系动词be
be为连系动词，中心词义是"是"，句型为"主＋系＋表"结构。be的形式常用am, is, are(现在式)；was, were(过去式)；will/can/may/must be(助动词/情态动词＋原形)；have/has/had been(助动词＋过去分词)等。如：
To help animals is helping people.(一般现在时)
The twins were very busy yesterday.(一般过去时)
It will be sunny tomorrow.(一般将来时)
She has been ill for over a week.(现在完成时)
功能二，助动词be
助动词be，无词义，辅助主要动词一起在句中作谓语动词。用法如下：
1. be+doing：构成进行时态，有现在和过去两种进行时态。如：
The girls is reading and copying the new words now.
Young Tom was always asking questions and trying out new ideas.
2. be+done：构成被动语态(主语是动作的承受者，done必须是及物动词)。如：
Tea is grown in my hometown.(一般现在时的被动语态)
This building was built three years ago.(一般过去时的被动语态)
Our classroom has been cleaned and tidied already.(现在完成时的被动语态)
How could this kind of cakes be made in your home?(含情态动词的被动语态)
That is a day never to be forgotten.(动词不定式的被动语态)
3. be+going to do，表示"打算或将要做某事"，be有现在和过去两种形式。如：
We are going to plant trees in the park.
I didn't know if she was going to come here.
4. be+to do，表示"按计划安排将要做某事"。如：
The new shop is not to be opened till next Monday.
One night an angel came to Mary and told her that she was to have this special boy.
功能三，there be
there be句式为：there be+主语部分＋状语部分，表示"某处存在某物"，be常用现在时，过去时和将来时等。如：
Oh, cool! And there are many things to see. There is even a deer park in Sanya.
There are about 80 pyramids in Egypt.
Will there be a football match in your school next week?
功能四，实义be
可以将be视为实义动词，因为它具有实际的词义，如"成为;做;发生;举行;逗留;到达"等。如：
His daughter wants to be a doctor for animals in her twenties.
Kate's birthday party will be at half past six this evening.
Jim has been in China for more than two years, but he has not yet been to Yichang.
