Translation:He had demonstrated that he can talk.
Perhaps, but "had" with "can" makes no logical sense. How could one demonstrate in the past an ability that he possesses now? At best, he could demonstrate the ability at that point, which is no guarantee that he has retained it.
The other way around is still logical: "I have demonstrated that I could juggle." I can't juggle any more, but I may have just shown a 15-year-old video that proves it.
Here are two links indicating that this sentence is perfectly fine as long as the meaning/context is as you describe.
Agreed. Even if it were about the present, the past tense of the auxiliary "had" determines the tense for the whole sentence, so then it should be "could". To refer to the present, it would be "He has demonstrated that he can ..." where the demonstration is still in the past, but making it relative to the present, not to another past.
I think conditional is only used when the verb before "that" is something like "wish" that suggests that the part following the "that" might not happen. In this case, the verb is "demonstrated" which means what follows the "that" is true.
I was going to say that I think using present tense is wrong, but then I re-read what I quoted a while ago (see my comment starting "I was thinking the same thing"). So, based on that, "He demonstrated that he can talk" means that he can still talk, while "He demonstrated that he could talk" means that he could talk in the past, but it doesn't say anything about whether he can talk now. <<gasp>>
I was thinking the same thing, and I tried to find a reference for a rule that says the part following "that" has to be in the past if the part before "that" is in the past.
But I was remembering the reported-speech rule. Tom said "I can talk". Tom said that he could talk.
In our sentence, the part starting with "that" is a noun clause, and it doesn't have the same "one tense back" rule that reported speech has.
I found this discussion: http://bogglesworldesl.com/askthomas_noun_clause1.htm
Here's a quote from that discussion:
>Now here's where it starts to get tricky to teach. How does a student know what tense to put the verb in the noun clause. Here's what I mean:
>At yesterday's meeting, the teachers all agreed that teaching noun clauses is/was a difficult problem.
>I don't know about you but I have to think twice about which verb to choose. In most cases, any tense of the verb in the noun clause is grammatically acceptable, but it often changes the meaning in ways that are too subtle to teach to your students.
>The teachers agreed that teaching noun clauses is difficult implies that in general noun clauses are difficult to teach.
>The teachers agreed that teaching noun clauses was difficult implies that it was a difficult at that particular time when they taught it.
My understanding is that they are both equally correct, but that people in different countries/regions prefer one or the other. To me (central Canada), "proven" sounds a bit more natural.
This site http://english.stackexchange.com/questions/68380/is-proven-very-old-fashioned quotes the OED as saying the original past participle was "proved", but "proven" was later introduced by analogy with words like "cloven".
the confusion of the present puede with the past "could" (which is correct) happens bc the tense of the ability is in relation to the "habia" "had" which casts the whole sentence into the past, which the English expresses with "could" and the Spanish, evidently with "puede", but the tense of the ability to speak is Past.