I came across the following example from the textbook Advanced Grammar in Use by Martin Hewings.

To say that we think a past situation actually happened, we use will have + past participle:

As it was cloudy, few people will have seen last night’s lunar eclipse.

Why isn't the past simple or past perfect used in the aforementioned example?

As it was cloudy, few people saw last night’s lunar eclipse.

marked as duplicate by Kris, tchrist, anongoodnurse, Mitch, Centaurus Aug 13 '15 at 23:01

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  • Your past simple is used to state a fact, and not what we think happened. You actually clearly say that yourself. What is unclear about the difference between "I think that X happened" and "X happened (it's a fact"? – oerkelens Aug 9 '15 at 9:36
  • If I think that situation HAPPENED, doesn't it mean that it is a fact? – kacherese Aug 9 '15 at 9:47
  • @kacherese Only in the minds of deluded people. – Joost Kiefte Aug 9 '15 at 9:50
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    This question belongs on English Language Learners – Kris Aug 9 '15 at 10:09
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    @Kris I believe the grammar book is not far off in stating that this construction implies that the speaker thinks there is a (high) possibility that something happened. Of course, that is very different from stating that it is a fact. – oerkelens Aug 9 '15 at 10:18

Because it was cloudy last night, few people saw the lunar eclipse.
Because it was cloudy last night, few people will have seen the lunar eclipse.

In what way is the speaker of the second sentence changing the statement, by using a future perfect?

The speaker is making a kind of prediction. Time will show that few people saw it. In the very near future, people will be saying they didn't see it. The future perfect there is a very mild form of "Mark my words". We have no reason to expect otherwise. One could even add "I expect" or "one expects" to the sentence:

Because it was cloudy last night, I expect few people will have seen the lunar eclipse.

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