I have been reading Meaning and the English Verb (Leech, Geoffrey N. 2004). I like its systematic treatment on tense, aspect, mood, and modality. I think it's a must-have for any advanced ESL learners. It's just amazing and extremely accessible, really a perfect complement to Practical English Usage by Swan.
But only one thing I still feel uncertain about is whether could can be used alone without adding "if-clause" to express irrealis thoughts. I know it is true of would(e.g. it would be fantastic to get to the moon), but I'm not sure it's also true of could or might.
Meaning and the English Verb §183
HYPOTHETICAL POSSIBILITY. Used hypothetically, could and might are substitutes for may in expressing factual possibility (see §121):
There could be trouble at the World Cup match tomorrow. | The door might be locked already. | Our team might still win the race.
The effect of the hypothetical auxiliary, with its implication ‘contrary to expectation’, is to make the expression of possibility more tentative and guarded. Our team might still win the race can be paraphrased ‘It is barely possible that…’ or ‘It is possible, though unlikely, …’.
Meaning and the English Verb §186
To conclude, the following sentences illustrate this multiplicity with examples of seven different meanings of could:
#1, #2 and #3 are omitted here.
#4. Hypothetical equivalent of can (= ‘possibility’) (cf. §176)
The house is one of the most beautiful that could be imagined.
#5. Hypothetical equivalent of can (= ‘ability’) (cf. §176)
Do you know anyone who could repair this clock for me?
#6. Hypothetical equivalent of can (= ‘permission’) (cf. §176)
I’d be grateful if I could borrow your electric drill.
#7. Tentative equivalent of may (= ‘factual possibility’) (cf. §183)
The weather has been terrible up there in the mountains. You could find climbing very difficult.
The last meaning is rather more anomalous than the others, as it shows could extending its range of meaning into the epistemic territory of ‘factual possibility’ which is the domain of may, not can.
I would think Leech is making clashing points here. Isn't the usage of could in the three examples in §183 identical with that in #7? If so, why does Leech say #7 is more anomalous? or, to put it another way, why does Leech separate #7 from #4 and #5?
I would think #7 is the most common usage of could. Is the usage of #7 derived form #4 or #5? For #4 and #5. I would think they are just the same as:
The house is one of the most beautiful that can be imagined.
Do you know anyone who can repair this clock for me?
What are the unexpressed conditionals implied in #4 and #5 respectively? If we used can, would any nuances be suggested? Why bother to use hypothetical mood there?