tdol said:
You can't have is perfectly normal BE. The positive is not used, we would use must have instead. ;-)
CitySpeak said:
I don't understand how "You must have seen him." is used as the opposite of "You can't have seen him."
"The
positive (i.e. You
can have seen him) is not used, we would use must have instead." In other words, speakers do not use 'can have -en' forms, which tdol refers to as 'The positive' form of 'can't have -en' because it lacks negation, 'not'.
To me, You can't have seen him = You couldn't have seen him.
Moreover, to me, the
opposite , or
affirmative of 'can't have' is 'can have':
Negative: You
can't have that cake. (means, impossible)
Affirmative: You
can have that cake. (means, possible)
But there is no 'can have -en',
Negative: You can't have seen him. (means, impossible, OK)
Affirmative: You can have seen him. (means, possible, Not OK)
"You can have seen him is not Ok because 'can' must agree in Time with 'seen'==> "You
could have seen him. In the case of "You can't have seen him", which is Ok, it differs in that the adverb 'not' serves to sever the relationship between 'can' and 'seen', like this,
a. You can [do this] (e.g. You can [have the cake])
b. You
can [not have seen him]. S+
V+O
In b. the "not have seen him" functions as the object of the verb "can". compare, c. below, wherein "not have seen" functions as the verb phrase and "him" functions as the object of that phrase:
c. You
could not have seen [him]. S+
V+O
Let's compare the two,
d. You can
not have seen him.
object
e. You could not have seen
him.
object
The difference between d. and e. has to do with structure. Some speakers view "can" as a verb (as in d.), whereas other speakers view "can" as a modal (as in e.) If speakers view "can" as a modal, then "can" must agree in Time with "seen" (i.e. could have seen); if speakers view "can" as a verb, then "can" need not agree in Time with "seen" because "seen" is not functioning as a verb; it's functioning as an object.
