tips: could

1)We use COULD (not can) for actions that are not realistic. For example: O I'm so tired,I could sleep for a week. (not I can sleep for a week)
Compare can and could:  I can stay with Sarah when I go to Paris, (realistic). Maybe I could stay with Sarah when I go to Paris, (possible, but less sure). This is a wonderful place. I could stay here for ever, (unrealistic).

2)We also use COULD (not can) to say that something (a situation or a happening) is possible now or in the future. The meaning is similar to might or may.
The story could be true, but I don't think it is. (not can be true) O I don't know what time Lisa is coming. She could get here at any time.
Compare can and could: The weather can change very quickly in the mountains, (in general). The weather is nice now, but it could change, (the weather now, not in general).raymond murphy grammar in use