英语单词在什么情况下要变形？

看哪一类词而定，名字变复数，动词变三人称单数，变过去式，分词，以及名词变动词，名词变形容词，动词词变名词，形容词和副词的互变等词形都会发生相应的变化

. 一般过去时的定义
一般过去时表示过去某一时候或某一段时间所发生了的事情或存在的状态。常与过去时间yesterday, this morning, just now, a moment ago, in May, last night / year / week, once upon a time, the other day, before …, when – clause, in the past连用。如：
What did you do yesterday? 昨天你干了什么？
I met Lin Tao this morning. 今天上午我会到了林涛。
I was there a moment ago. 刚才我在那儿。
2. 一般过去时的应用
(1) 表示过去某时所发生的动作或存在的状态。如：
Liu Ying was in America last year. 刘英去年在美国。
Jim rang you just now. 吉姆刚才给你打了电话。
(2) 表示过去经常或反复发生的动作。常接时间副词often, usually, always, sometimes, every day / week, etc. 如：
We often went out for a walk after supper. 我们过去常在晚饭后散步。
We usually played together. 我们通常一起玩。
3. 一般过去时对谓语动词的要求
一般过去时的谓语动词要用动词的过去式。动词过去式的构成分规则变化和不规则变化两种形式，不规则变化通常需要逐个记忆，规则变化则遵循以下原则：
(1) 一般在动词后加-ed。如：play—played, offer—offered, weigh—weighed, destroy— destroyed, sign—signed.
(2) 在以字母e结尾的动词后，只加-d。如：like—liked, provide—provided, hate — hated, date—dated。
(3) 在以“辅音字母+y”结尾的动词后，则改y为i，再加—ed。如：supply—supplied, fly—flied, study— studied.
(4) 在以单短元音的重读闭音节结尾且，末尾只有一个辅音字母的动词后，双写最后一个辅音字母，再加-ed。如：plan—planned, refer—referred, regret—regretted, ban—banned.
4. 特别说明
有些动词的过去时，如：expect, hope, intend, plan, wanted 等一般过去时，后接不定式的完成时；或它们的过去完成时接不定式的一般式，都可表示过去未曾实现的意图、打算或希望。如：
I hoped to have been invited to his wedding party. —I had hoped to be invited to his wedding ceremony. 我本希望他来邀请我参加他的婚礼。
I intended to have joined their games. —I had intended to join their games. 我本打算参加他们的比赛。

时态详解：过去将来时
1. 过去将来时的定义
过去将来时主要用于表示过去某个时候看来即将发生的动作或存在的状态。如：
He said he would come here next Friday. 他说他下周星期五来这儿。
I knew that he would help us when we were in trouble. 我知道当我们陷入困境时它会帮助我们。
2. 过去将来时的结构
(1) would + 动词原形。如：
She told us that she would try her best to catch up with other classmates this term. 她告诉我们说她将一切努力在本期赶上其他同学们。
When you asked Li Lei for help, he would never refuse you. 过去当你请她帮忙时，他绝不会拒绝。
(2) was / were going to + 动词原形。如：
He told us that he was going to attend the meeting. 他告诉我说他要参加那次会议。
He said that I was going to be sent to meet her at the railway station. 他说将要拍我去火车站接她
(3) was / were to + 动词原形。如：
The building was to be completed next month. 这座建筑改在下个月竣工。
Li Lei was to arrive soon. 李蕾很快就要到了。
(4) was / were about to + 动词原形。如：
We were about to leave there when it began to rain heavily and suddenly. 就在我们要离开时，天突然下起了大雨。
He was about to have lunch when the bell rang. 就在他要吃中饭的时候，门铃响起来了。
(5) was / were +现在分词。如：
He was leaving the next day. 他第二天要走了。
We were informed that the leaders were coming to our school soon. 我们接到通知说领导们很快要来我们学校。
3. 过去将来时的用法
(1) 过去将来时，一般用于主句为过去时的宾语从句中。如：
He said he would stay with us. 他说他要与我们呆在一起。
He said he would never go there again. 他说他绝不会再去那儿。
(2) 过去将来时，用于虚拟语气中，如：
If I were you, I would not do that. 要是我是你的话，我就不会那样做。
If he were here, he would show us how to do it. 如果他在这儿，他就会向我们展示该如何做了。
4. 拓展
was / were going to + 动词原形；was / were to + 动词原形；was / were about to + 动词原形等结构都可表达当时一种未曾实现的意图或打算。如：
The conference was going to be held the next month. 会议下个月开。
We were to have our class at eight. 八点我们该上课了。
I was about to tell him about it when Wu Dong got in. 就在我要告诉他时，吴东进来了