They were about to get down the hill when it began to rain

They were about to get down the hill when it began to rain
为什么不是
They were about to get down the hill when it was beginning to rain
他们正要下山,这时天突然开始下起了大雨（begin）

e doing sth.是现在进行时,并且begin作为短暂性动词,通常没有ING形式.
再问： 他们正要下山，这时天突然开始下起了大雨（begin） 完形填空： 正确句子是They were about to get down the hill when it began to rain 我的答案是They were about to get down the hill when it was beginning to rain 我不懂为什么正确答案是那样的 when后面不是应该跟现在进行时吗
再答： 从语法角度，be doing 确实是现在进行时，但begin这个词是开始的意思，它是短暂性动词，开始就是开始，不存在一直开始吧？所以不能把开始变成现在进行时。 从口语角度，直接说when it began to rain,你也可以直接记住： begin to do = begin doing意思是开始做。。。