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We were checking some exercises and I wanted to know what's the correct modal to use:

Gemma is away in Australia all summer. You ___ (can / see) her yesterday!

The answer is can't have seen, which is I selected. However, some of my classmates were saying that couldn't have seen is also an option. Is this valid too?

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Yes, both are valid, with only very slightly differences in meaning (for most purposes they are all but identical). In fact, I would find couldn't have seen more appropriate, especially since the event in question clearly occurred yesterday.

This is an example of a conditional perfect tense (e.g. would have been) with a modal verb (can/could) replacing the standard would.

Interestingly, the structure "could have [past participle]" appears to be fairly rare when discussing these kinds of modal constructions. For example, this British Council page and this SpeakSpeak page both discuss the could have formation but not the can have formation (at least, not in its positive sense; can't have is discussed). Wikipedia mentions that can have been is a "rarer alternative" to could have been, but that section is listed without source, so take from that what you will.

Beyond the grammar, couldn't in this context appears to be more common in my experience, and is evidenced in this Ngram.

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  • Yes, I thought the same. Despite having yesterday as a time expression, the natural way is to use couldn't have seen, but I discarded this option since at the beginning we have Gemma is away and not Gemma was away. The book points for can't have seen, but if two options were available, why wouldn't they put them as well?
    – Schwale
    Nov 28, 2015 at 15:00
  • @Nonnal: Good answer, but my ear prefers the directness of "can't." Ale also makes a good point in the comment above. One aside: Skype has rendered this sample sentence obsolete. Nov 28, 2015 at 15:00
  • @Ale Since Gemma is away "all summer," this is less about the present tense and more about a statement of her ongoing absence. "Gemma was away all summer" implies that she no longer is away. No clue why they would put two correct answers as options, but could have seen is clearly preferable to my eye.
    – Nonnal
    Nov 28, 2015 at 16:17
  • @MarkHubbard Fair enough. I wouldn't be surprised if the can't-vs-couldn't distinction has regional variance as a factor. I've edited the post and added some references to support my position that couldn't is more common and preferable in the general use case, but I wouldn't be surprised to discover that can have seen is more common in certain geographies.
    – Nonnal
    Nov 28, 2015 at 16:20
  • @Nonnal Agreed, and thank you for the additional information. Nov 28, 2015 at 16:42
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I think there is a grammar versus usage issue here.

Strictly speaking, your book is correct and your classmates are wrong. The sentence is a past probability in which the construction is:

modal verb + have + past participle = can't have seen

The justification is that seen marks the sentence for the past, so there is no need to mark the modal as well.

But... couldn't have seen sounds natural enough to me, and if comments below Nonnal's post are any guide it sounds natural to others too. My hunch is that using couldn't instead of can't marks for irrealis, often in a conditional sentence such as "If she is in Australia, you couldn't have seen her yesterday", so a lot of people subconsciously see this a a conditional sentence because not seeing her is true only if her being in Australia is true.

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