[riverkid]

MikeNewYork:The construction exists in English, but I have to disagree with Riverkid about the implications. Modal verbs are different in many respects from other English verbs, but they still show tense. Modals exist in pairs

(present/past):
wiil/would
can/could
shall/should
may/might

Modals exist in pairs as Historical Present Tense forms and Historical Past Tense forms. In the English of today they are tenseless forms. We know this because they operate in all time situations.

In order to understand that, you have to understand that a tense is a construction that is often related to time, but a specific tense can refer to a time other than what is named in that tense (past, future, etc.) and the past tense can be used for things other than past time, such as politeness or hypothetical situations.

That's true, but the use is much more limited for the lexical verbs. Modals are special verbs that carry modal meaning into sentences. Tense is marked by other considerations.

The constructiuon you are referring to is "can have + past participle". We also have "could have + past participle". The first has a present tense modal and the second has a past tense modal. So let's look at the difference.

In these two sentences we have the negative form of the tenseless modals 'can' and 'could'.

He can't have eaten all five pizzas by himself.
He couldn't have eaten all five pizzas by himself.

Both sentences express the speaker's disbelief with reference to the action.
They have almost identical meanings. The only difference is where the speaker places himself. The first sentence puts the speaker's disbelief in the prersent. (Even now) I can't believe that.... The second sentence is just a statement about the past, so it uses a past tense modal.

My learned colleague tries to make a distinction where none exists. The speaker isn't placing himself anywhere. Both speak to the same [possibly tentative] past action.

The second sentence also contains the meaning "Even now, I can't believe that ...".

Clearly, both discuss a past action. What is the difference? <can> being more direct, a modal of greater reality, shows that the speaker is not willing to allow as much doubt for the situation as with the use of 'could', which is a more tentative modal.

These examples don't say very much about tense because the meanings are too close.

The meanings of can and could are very close. It's the emotive sense, the nuances that make the difference. But both modals operate in all time situations. That's what makes them tenseless.

Let's look at another example of "can" versus "could".

Ten years ago, I could run a marathon; today I couldn't/can't.

In the first clause, "could" is obviously past tense. The time is clearly in the past (ten years ago). One cannot substitute "can" for "could", because the time is in the past.

That makes as much sense as saying one cannot substitute shall or should or may. There can be no substitution because of modal meaning.

A: Someone took my pen.
B1: Brian could have it.
B2: *Brian can have it.* [* denotes ungrammatical for the context because the modal meaning of "it's possible" simply isn't there.]

Here, in an obviously present time situation, we cannot substitute purported present tense 'can' for 'could'. Why? Nothing to do with tense. The reason, as noted above, but bears repeating, has to do with modal meaning.

The substitution argument makes no sense.

Can we use 'can' in this past time situation. Absolutely.

That fat old fart, he can't have run a marathon back then. He was as heavy then as now. What a big BSer!

Ten years ago, I could run a marathon; today I couldn't/can't.
The second clause is obviously in the present. Therefore, one can use either modal.

And why pray tell, can we all of a sudden use a "past tense" modal here?

The present tense "can't" is more concrete. It gives the impression that the speaker has tried and failed. One can also use "couldn't". The past tense form gives the impression of a hypothetical situation. The past tense of verbs is often used for that reason. The speaker may have chosen "couldn't" because he has a bad hip or has gained 50 pounds, either of which would be likely to interfere with a 26 mile run.

Interesting conjecture, Mike but you mislead. Both modals state the same meaning. Using <can't> doesn't mean that the speaker has tried and failed. Wouldn't you think that a person with a bad hip or one that has gained 50 pounds would tend to use the more concrete <can't>.

Again, not necessarily but it's certainly possible. The differences relate to speaker choice, which you've taken great pains to illustrate to confuse the fact that a purported past tense is being used in a present tense situation.


In any case, modals give the reader/listener a window into the mind of the speaker. Modal verbs are extremely important and they do not need to be strippped of "tense" to accomplish their tasks.

We don't need all the mental gymnastics to defend an obvious language miscalculation. Clearly, this is what makes modals tenseless; it's what modals can and do do, operate in all time situations.