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:
- I could do the job [then].
- 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:
- 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.
Other references on this issue can be obtained with this Google Books search:
"could" "was able to" "past time"