What is Could ?
Could is modal(verb) 1. meaning ‘was’ or ‘would be able’ to do something The old lady fell down and couldn’t get up. You could still catch the train if you ran. 2. meaning ‘was allowed’ to do something The policeman said we could go into the house. 3. used in asking Could you pass me the salt, please? Could you shut the window? 4. meaning ‘might happen’ The new shopping centre could be finished by Christmas. 5. used in making a suggestion You could always try borrowing money from the bank. (NOTE: The negative is could not, usually couldn’t. Note also that could is the past of can; could is only used in front of other verbs and is not followed by the word to.)
source: Easier English, Student Dictionary Upper Intermediate Level