AnonymousDo we have to use a past perfect tense when an event or action has occurred before some action or event, except for cases where sentences have words like 'before' and 'after'?
No. We say:
When he arrived, he noticed that the window was open. Even though the arriving happened before the noticing, there is no past perfect in this sentence. Often just the order of the narrative indicates the order of the events in time.
Others:
Gerald awoke suddenly that morning and jumped out of bed.
The plane taxied down the runway and took off.
As soon as I saw that the meat was spoiled, I threw it away.
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until doesn't necessarily require a past perfect either. In fact, to my ear, it often sounds better without a past perfect in the sentence.
Until I came here, I had given it some thought.
Until I came here, I gave it some thought.
Peter didn't say anything until he was ready.
Peter hadn't said anything until he was ready.
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The reasoning behind your other example is different. To preserve the 'sequence of tenses' you only have a limited number of choices.
Either you are talking about how he looks now or how he looked in the past as the result of some earlier event that caused him to look a certain way.
He looks as if he has seen a ghost.
He looked as if he had seen a ghost.
Or you are talking about how he looks now or how he looked in the past as the result of a state or activity happening at the same time.
He looks as if he is staring at a ghost.
He looked as if he was staring at a ghost.
In any case, the sequence of tenses is usually observed in such statements.
as if is nevertheless very flexible, so you will find a variety of tense combinations with it.
CJ