These modals express possibility and ability.
Can indicates ability. Could indicates ability with an option.
Example:
- I can do it. (The subject ‘I’ is sure about his/her ability)
- I could do it. (The subject ‘I’ is not sure about his/her ability)
- They cannot do it. (present)
- They could not do it. (past)
Can & could also indicate possibility.
Example:
- The temperature can rise this month.
- They can’t go too far by now.
- It could rain later.
May and might both indicate possibility but might can suggest that there is less possibility than may.
Example:
- It may rain later.
- It might rain later.
- They may come back.
- They might come back.