COULD U

"Could" is 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."

Examples:

A. Extreme rain could cause the river to flood the city. - possibility

B. Nancy could ski like a pro by the age of 11. - past ability

c. You could see a movie or go out to dinner. - suggestion

D Could I use your computer to email my boss? - request

E. We could go on the trip if I didn't have to work this weekend. - conditional

Using "Could" in Present, Past, and Future

Most modal verbs behave quite irregularly in the past and the future. Study the chart below to learn how "could" behaves in different contexts.

Modal Use Positive Forms
1. = Present 2. = Past 3. = Future Negative Forms 
1. = Present 2. = Past 3. = Future You can also use:

could - possibility : u can also use may and might.
Positive. 
1. John could be the one who stole the money.
2. John could have been the one who stole the money.
3. John could go to jail for stealing the money. 

Negative:
1. Mary couldn't be the one who stole the money.
2. Mary couldn't have been the one who stole the money.
3. Mary couldn't possibly go to jail for the crime.

could - conditional of can

Positive: 1. If I had more time, I could travel around the world.
2. If I had had more time, I could have traveled around the world.
3. If I had more time this winter, I could travel around the world.

Negative: 1. Even if I had more time, I couldn't travel around the world.
2. Even if I had had more time, I couldn't have traveled around the world.
3. Even if I had more time this winter, I couldn't travel around the world.

could - suggestion: 1. NO PRESENT FORM
2. You could have spent your vacation in Hawaii.
3. You could spend your vacation in Hawaii. 

NO NEGATIVE FORMS 

could - past ability: ( be able to )

1. I could run ten miles in my twenties.
2. I could speak Chinese when I was a kid.

"Could" cannot be used in positive sentences in which you describe a momentary or one-time ability.

Yesterday, I could lift the couch by myself. - Not Correct

Negative:
1. I couldn't run more than a mile in my twenties.
2. I couldn't speak Swahili.

"Could" can be used in negative sentences in which you describe a momentary or one-time ability.

Yesterday, I couldn't lift the couch by myself. - Correct

could - polite request: Requests usually refer to the near future. ( u can use can and may instead of could ) 

Positive: 1. Could I have something to drink?
2. Could I borrow your stapler?

Negative:
1. Couldn't he come with us?
2. Couldn't you help me with this for just a second?

REMEMBER: "Could not" vs. "Might not"
"Could not" suggests that it is impossible for something to happen. "Might not" suggests you do not know if something happens.

Examples:
1. Jack might not have the key. Maybe he does not have the key.
2. Jack could not have the key. It is impossible that he has the key.