Be about to

The be about to structure is a very commonly used multi-word verb form, in this case, a semi-auxiliary (i.e. it always needs a main verb (unless, of course it's being used as a short answer, in which case it's used like any other auxiliary - see below).

Future

 * it's very commonly used to express future intention - similar to the "be going to" structure I'm going to make a nice cup of tea, but more immediate: I'm about to make a nice cup of tea.

Past

 * In the same way that "be going to" is often used in the past, I was about to make a cup of tea is a typical expression referring to an intention that didn't materialise in the recent past (I was about to call you) or has been temporarily interrupted I was about to make myself a cup of tea but now that you're here I'll make a teapot.

Short answers
Like any other auxiliary, when used as a short answer to a question, it's used without the main verb: Have you done your homework? - I was about to.