Could not (also couldn't)

"Could not" is a modal auxiliary verb of Could. There is no –s in the third person singular.  "Could not" suggests that it is impossible for something to happen.
 Ex: Jack could not have the key. It is impossible that he has the key.
Lisa couldn’t come to the party because she was ill.
Women couldn't vote until just after the First World War. General possibility

• Why couldn't you attend the meeting yesterday?
• We found that we couldn't depend on our guide.  

Could not (also couldn't) shows past inability.

 

• I could not understand a word, but I kept smiling.
• She spoke in such a low voice that most of us could not hear her.
• We found that we couldn't depend on our guide. 

As the past equivalent of can (can't)  in indirect speech.

Ex: She said that she could not climb up the hill.