We could not imagine how the Inca could have built such a large city on top of a high mountain.

I don't think could build can replace could have built in the sentence above, but I am not sure why not.

What is the difference between could + have p.p. and could + infinitive?

I learned from a previous post in this site that could + have p.p. could mean inference or deduction of past possibility. And could + infinitive could mean present or past possibility.


The sentence

  • We could not imagine how the Inca could have done that.
    (let's just concentrate on the modals here, OK?)

is grammatical, and means the same as the sentence

  • We could not imagine how the Inca could do that.

Both refer to past times, but the first one overtly specifies the time of the Inca's actions as prior to the time of our failure to imagine them, while the second one doesn't.

In a case where the context makes the order of events obvious (the Inca's actions are in the far past, whereas we're still alive), it's not necessary to specify it overtly.

If you do specify that order when you don't need to, you draw attention to it. This may be what you want to do; or it may be unnecessary detail that slows down the reader/listener's understanding of your point.

The choice is, as usual, up to the writer/speaker, who has a better idea than anybody else of what they want the reader/listener to understand, and how to achieve it.

Neither sentence is subjunctive, by the way. Just more modal auxiliary usages, that's all.

  • I liked this answer and I got a question for you! How should I know here "could" is the past form of can not a present tense modal impying a week possibility? Also, why shouldn't I understand the sentence as the author is not sure if they actually built the city? I mean why shouldn't I get "could have built" as "they might build"; like they could have called me but they didn't! – Cardinal Jun 10 '18 at 23:36

The first sentence is in the subjunctive mood. The sentence does not cast doubt on the fact that the Inca built the city, it merely says that it was difficult for the writer to imagine how this was accomplished.

you can replace 'could have built' with 'could build' but the new sentence could be interpreted as casting doubt on the fact that the Inca built the city.

I agree with what you learned from a previous post.

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