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One is known as the attitudinal past , that is, the past tense is associated with the present time in independent clauses expressing a question, request or suggestion. Its effect is to make the question / request / suggestion less direct, implying a polite, somewhat tentative attitude on the part of the speaker.
第一种情况是表示 婉转口气 。
A: Did you want me?
B: Yes, I wondered if you could give me some help.
The other is what we call the hypothetical past. In this use, the simple past refers not to a fact but to a non-fact, and is typically found in that-clause, following such construction as “It’s time…”, “I wish…”, “I’d rather…”, etc, and in adverbial clauses of rejected condition, ie a condition which is not likely to be fulfilled.
第二种情况是在 It’s time…, I wish…, I’d rather … 等结构后面的 -that 分句中，以及在某些条件句中，表示主观设想 。
The use of past progressive (was / were + -ing participle) has much in common with that of the present progressive, only the time reference being pushed back to the past, often overtly expressed by a time- when adverbial. The following are the uses of the past progressive:
过去进行体 是由助动词 be 的过去时形式 (was, were) 加 -ing 分词构成。过去进行体的用法与现在进行体的用法有不少相似之处，只是时间向过去推移而已。过去进行体可用于：
1. To denote an action in progress at a definite point of period of past time.
This is the most common use of the past progressive. In this use, the past time reference is usually indicated by a temporal adverbial or implied by the context. In the past progressive, the notion of incompleteness is more clearly indicated than in the present progressive.
It is based on this use that the past progressive may have the effect of surrounding a particular event by a temporal frame, or serve, at the beginning of a text, as the background in past time narrative.
A thirsty Ant was climbing down a blade of grass that grew beside a spring. She was trying to reach the water so she could take a drink. Unluckily she slipped and fell into the spring.
Now a Dove was sitting on a branch over the water. She saw the Ant fall in and was filled with pity. Quick as a wink she pulled off a leaf and let if fall into the spring. The little raft settled down on the water right beside the drowning Ant. The Ant climbed on the leaf and was soon safe on shore again.
But what did she see? Hidden behind a bush, a hunter was spreading his net. He was going to catch the Dove. “No!” the grateful Ant said. “You shall not take the bird that saved my life!” And with all her might she bit the hunter on his bare foot. With a cry the hunter dropped his net, and the Dove flew away to the wood. One good turn deserves another.
As in the case of the present progressive, the past progressive can also collocate with such adverbials of frequency as always, constantly, continually, for ever , to express emotional feelings, especially feelings of annoyance or disapproval on the part of the speaker.
过去进行体也可与 always, constantly, continually, for ever 等状语连用表示感情色彩 。如：
The chief use of the past perfective progressive has also something in common with the “unfinished” use of the past perfective, and, therefore, in many cases, these two forms can be used interchangeably, though in colloquial speech, the past perfective progressive is more frequently used than the past perfective.
Rarely can two past perfective forms co-occur in a sentence to show time sequence of two actions except for the kind of sentence comprising a clause introduced by the subordinator by the time. Even in this kind of clause, the simple past is more frequently used than the past perfective.
能同时使用两个过去完成体的场合极少，下面带有由 by the time 引导的时间状语分句的句子是少数例外中的一个。
By the time I ( had ) recovered from the shock, he had disappeared .
We had got everything ready by the time they arrived .
By the time I got downstairs, the telephone had stopped ringing.