The rain began before sunrise.

雨是在日出前开始下的。

《柯林斯英汉双解大词典》

A light rain began to fall.

小雨开始飘落。

《牛津词典》

The boy was busy doing his work when a heavy rain began.

那男孩正忙着干活，突然下起了大雨。

youdao

The march began under gray skies and light rain symbolically at the Holocaust Museum, where the murder of six million Jews in World War II is documented and remembered.

VOA: standard.2010.05.23

Then, 12 minutes into Obama's speech, the skies opened up and the heavy rain began to fall.

NPR: Obama Taking Nothing For Granted With 1 Day Left

And then, as rain began falling three laps from the finish, the drama reached a new height.

CNN: Extraordinary F1: the final race in pictures

As the rain began to lash down the game turned scrappy with Tim Cahill and Sturridge off-target with half-chances.

BBC: Gary Cahill scores for Bolton