我正准备去干某事.用英语怎么说?

I am about to do sth,注意,后接when引导的并列句,表意外,突然之意思.
如I was about to have supper when the phone must ring.我正要吃晚饭时偏偏电话响起来.
He was about to get married when both his parents died in an accident.他马上就要结婚了,谁知父母双双死于事故.
还有：be about to do后不可以使用时间状语,如下面得句子是错误的
We are about to leave here in an hour.(错误)
你的句子可译为：I'm about to cut the grass in the field.