Could
We can use could + infinitive to talk about a general possibility in the past (compare with the use of 'can' above):
Prices could be high in the sixteenth century.
This is not used to talk about specific possibilites in the past (instead we use could + have + past participle):
He could have been working late (not: 'could be'. As this is a specific possiblity, 'could be' is present tense)
