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He told me he would give me the answer after the trip. Was there something he could only know by then?

I don't know why but it sounds a little weird to me. Anyway, to make sure I searched on Google. As you can see, there's only 1 result.

Did I misspell something? Or use a wrong word?

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An equivalent phrase that is more idiomatic (Google says about 14,000,000 results) is "won't know until then", for example:

So Help Me God By Larry D. Thompson

She hasn't regained consciousness. I think we have the right antibiotics for now. We won't have the blood culture results back until at least tomorrow morning. It may be that there is another antibiotic that will be more effective to fight the infection. We won't know until then.

So consider this wording instead

He told me he would give me the answer after the trip. Was there something he wouldn't know until then?

A Google search for "wouldn't know until then" yields about 11,200,000 results.

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    But can I still use won't if the passage is in the past tense? – janoChen Apr 17 '15 at 8:58
  • @janoChen, good point, perhaps "Was there something he wouldn't know until then?" --- Fixed it. – amdn Apr 17 '15 at 9:01
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As opposed to "could have" (which is appropriate for other contexts - they are not equivalent), "could," while correct, has a much more limited use in past time contexts.

Many times you'll find replaced, when the past time is involved, as in:

  1. I could do the job [then].

by

  1. I was able to do the job [then].

This is what makes it quite rare, and makes us feel it awkward at times.

One important reason is removing the potential parsing uncertainty wrt time in the mind of the reader, created because the modal/subjunctive mood "could" is very frequently used in present time:

  1. Yes, I guess I could do the job [right now].

This time ambiguity is of course smaller when appropriate adverbials are used, still the users are wary about it.

However, there's no such reduction in past time use for "would," thus go for:

Was there something he would only know by then?

in this case.

A further important limitation on can/could, which practically proscribes 1 is described in the following very interesting reference:

An important point here is that could (like can) does not suggest that the action was carried out but only that the possibility was there. In fact, it is seldom used with a specific action except in the negative, for it would suggest that the action did not take place. If the event did in fact happen, we would not use could but did. This is why we do not say *‘I could have a good T time at their party yesterday’ if a good time was had. Rather we would use a factual statement, ‘I had a good time at their party’, showing that the speaker both could and did have a good time.

Although to be able to is a fairly accurate paraphrase of can in most circumstances, using it to describe a past action uncovers a clear difference; ‘was able to do X’ implies that X was done though with some effort probably, while ‘could do X' says only that it was possible, and with a specific action, implies that X was not done.

Realms of Meaning: An Introduction to Semantics By Th. R. Hofmann

Other references on this issue can be obtained with this Google Books search:

"could" "was able to" "past time"

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