"Él no puede haber leído este libro."
"Haber leído" is a compound infinitive, you can read of its uses here: http://www.elearnspanishlanguage.com/grammar/verb/perfectinfinitive.html :]
Thanks! Also, where did you go on Duolingo to answer this question? Clearly you aren't studying spanish...
Haha I figured that much! I was asking more of a Duolingo site question. Let's say I want to help people with their English questions when they ask about a specific question in the learning lessons. Where do I go to view new questions?
Oh, sorry XD You can only get into the English forum if you set Duolingo in a different language (Account - Settings - Learning Language).
Oh never mind, I just realise we have two "could"s in English, one is past tense and one is conditional. It's never occurred to me before!