Could, may or might convey the idea of future possibility. Of these, may expresses a stronger degree of certainty that an event will occur. For example: The temperature is dropping.

What is the future tense of could?

The use of 'could', 'would', or 'will be' all imply future tense. The past tense version would be: "You could not have made me happy, and I am convinced that I am the last woman in the world who could have made you so."

Could used for possibility?

"Could" is a modal verb used to express possibility or past ability as well as to make suggestions and requests. "Could" is also commonly used in conditional sentences as the conditional form of "can."