He used to drink alcohol.他过去喝酒。
（意味着他现在不喝酒了。喝酒这个动作终止了）
I used to take a walk in the morning.我过去是在早晨散步。
（意味着现在不在早晨散步了）
比较：
I took a walk in the morning.我曾经在早晨散过步。
（只是说明过去这一动作）

（7）有些句子，虽然没有表示过去确定时间的状语，但实际上是指过去发生的动作或存在的状态的话，也要用过去时。
I didn''t know you were in Paris.我不知道你在巴黎。
（因为在说话时，我已经知道你在巴黎了。这句话指的是说话之前，所以只能用过去时表示。实际上，这句话暗指：But now I know you are here.）
I thought you were ill.我以为你病了呢。
（这句话应是在说话之前，我以为你病了。但是现在我知道你没病）

一般过去时三变技巧:
一变：肯定句变为否定句
技巧1.当句中含有情态动词或助动词could，would，should等时，可直接在其后面加not构成否定句。例如：
I could get you a concert ticket. → I could not / couldn't get you a concert ticket.
技巧2.当句中含有系动词was，were 时，可直接在其后加not构成否定句。例如：
I was on the Internet when you called me. → I was not / wasn't on the Internet when you called me.
技巧3.当句中谓语是除情态动词、助动词、系动词was, were以外的动词时，在该动词之前加did not / didn't，动词还原，构成否定句。例如：
The famous singer sang some Chinese songs. → The famous singer did not / didn't sing any Chinese songs.

二变：陈述句变为一般疑问句
技巧1.移动词语的位置。将was，were, could，would，should等移到句首。例如：
He could pack his things himself. → Could he pack his things himself?
技巧2.添加助动词did。谓语是除情态动词、助动词、系动词was, were以外的动词时，在主语之前加did，动词还原。例如：
Mr Li looked very old. → Did Mr Li look very old?

三变：陈述句变为特殊疑问句
技巧1.确定疑问词：人who / whom，物what，地点where，时间when / what time，原因why，频率how often，长度how long，距离how far等等。例如：
They gave the concert last night. → When did they give the concert?
技巧2.辨认结构形式：疑问词+情态动词/助动词/ was / were / did +主语+...? 例如：
The accident happened near the station. → Where did the accident happen?