Dic: some usages of the modal ‘could’

2 [MODAL] You use could to indicate that something sometimes happened.

• Though he had a temper and could be nasty, it never lasted...
• He could be very pleasant when he wanted to.

 _CF. can: 4 [MODAL] You use can to indicate that something is true sometimes or is true in some circumstances.

• ...long-term therapy that can last five years or more...
• Exercising alone can be boring...
• Coral can be yellow, blue, or green.

4 [MODAL] You use could to indicate that something is possibly true, or that it may possibly happen. (= might)

• Doctors told him the disease could have been caused by years of working in smokey clubs...
• An improvement in living standards could be years away...

6 [MODAL] You use could to talk about a possibility, ability, or opportunity that depends on other conditions.

• Their hope was that a new and better East Germany could be born...
• I knew that if I spoke to Myra, I could get her to call my father.

.... Cobuild