General Characteristics (2) : can, could
Can and could are also used to refer to a general characteristic or quality
that may show itself from time to time:

  A house in London can cost a lot of money.
  He could be very unpleasant when he was angry.

Neigher of these sentences refers to an actual occurrence of the phenomena
referred to, and be able is not used as a substitute for can or could in such
sentences.

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