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I want to know that: does the perfect tense function it proper usage when it preceded by “to” infinitive.

I was taught that we couldn’t put “ago” with present perfect.

So, I came across a sentence such as the following:

  • These lions seem to have existed 70 years ago

Isn’t the same as?:

  • These lions have existed 70 years ago. incorrect

Warning: Cambridge Dictionary

We normally use ago with the past simple. We don’t use it with the present perfect:

I received his letter four days ago.

Not: I have received his letter four days ago. Incorrect


The source

  • Luskin’s team found that there are now only two habitats with viable populations, down from the 12 thought to have existed 70 years ago.
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  • ago is fine with the perfect infinitive. – Tᴚoɯɐuo Jan 30 '18 at 0:29
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I will try to explain this to myself, and maybe it will make sense to you :)

With the present perfect, the temporal origo is the speaker's Now, and we cannot use ago with the present perfect, because ago excludes The Now.

However, with the present perfect infinitive, the origo is free-floating (the infinitive being non-finite): it can be projected into the future or into the past:

To have existed in a world without elephants may well be the fate of people living only 50 years from now.

The Yeti is believed to have sent telegrams to Sasquatch 100 years ago.

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  • It definitely makes sense. I, totally, got your point of explanation that present perfect doesn’t act like it has a specific time when it comes with to-infinite. – Bavyan Yaldo Jan 30 '18 at 0:43
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"seem to have existed" has nothing to do with the present perfect. The sentence:

These lions seem to have existed 70 years ago

is equivalent to:

It appears that / Apparently these lions existed 70 years ago.

The perfect infinitive (formed by "(to) have + past participle") refers to the past and can thus take any adverbial typical of the past tense. Other examples:

  • He must have arrived yesterday. (I conclude/infer that he arrived yesterday.)

  • He happens to have arrived earlier than expected. (It turns out that he arrived earlier than expected.)

As you can see, the perfect infinitive accompanies modals or expressions that can indicate the speaker's current attitude or view in regard to a past event, and this enables the perfect infinitive to take past adverbials.

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  • The "past" of the perfect infinitive can be in the future, as it is a relative past. It will be a source of great pride for her to have argued before the Supreme Court . She might be arguing before the Court tomorrow. – Tᴚoɯɐuo Jan 30 '18 at 14:16
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These lions seem to have existed 70 years ago

The perfect infinitive is formed with to have + the -ed form of a verb: to have existed.

The perfect infinitive is used after another verb: seem in your first example.

Or, as in the source:

Luskin’s team found that there are now only two habitats with viable populations, down from the 12 thought to have existed 70 years ago.

The verb here is thought, followed by the perfect infinitive, and thought is the past simple that you see with ago.

This phrase has a relatively complex construction, with multiple components and verb conjugations:

Luskin's team found / (that) there are now only two habitats with viable populations, / down from the 12 thought + to have existed 70 years ago.

The first clause/phrase has the simple past found, the second has the present are and the third has the simple past thought with the perfect infinitive to have existed.

If it helps, the sentence could be rewritten as:

Luskin's team found that there are now only two habitats with viable populations. Luskin's team thinks that 70 years ago there were 12 habitats with viable populations.

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  • "thought" is not past tense but past participle and is most probably the abridged form of the relative clause "that are thought". The main verb in that verb phrase is then "are", which is present. What accounts for the use of "70 years ago" is the perfect infinitive "have existed", not the passive present from which the past participle derives. If the relative were "that **were"" thought", then the adverbial would be ambiguous: that was the belief 70 years ago, or the lions existed 70 years ago. – Gustavson Jan 30 '18 at 0:50
  • You are mistaken. Here thought is simple past. The past participle would need to be conjugated with to have. – Mick Jan 30 '18 at 1:13
  • "thought" is beyond any doubt the participle of the passive in that sentence: 12 habitats (that are) thought to have existed 70 years ago). – Gustavson Jan 30 '18 at 10:05

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