[Past time] The simple past and the past progressive

過去進行式：過去時間點，正在做什麼

長時間動作是進行式

短時間動作是過去式

example: When I walked into the living room yesterday, Grandpa was reading a newspaper and smoking his pipe.

The simple past V.S. the past progressive

過去簡單式

When it began to rain, I stood under a tree.

(The rain began 先發生, I stood under a tree 後發生)

(我站在樹下這個動作是發生在下雨之後 ) (我一定淋到雨)

過去進行式

When it began to rain, I was standing under a tree.

(I was standing under a tree 先發生，It began to rain 後發生)

the same meaning

when = at that time (這個時間點，所以不能用到進行式)

When the phone rang, I was sleeping.

while = during that time

The phone rang while I was sleeping.

by the time 那個時候