COULD

-Sirve para expresar habilidad en pasado: "When I was a child I could climb trees."
-Sirve para expresar posibilidad o imposibilidad en pasado: "Desert was so hot that we couldn't walk in the sand." 
-Sirve para pedir algo educadamente: "Could you pass the salt, please?" 
-Sirve para hacer especulaciones de posibilidad muy remota: "The wallet could be his, but i could be wrong".
-Sirve para hacer sugerencias: "We could play scrabble today."
-Sirve para reemplazar el can en estilo indirecto: " He said he could do it". 
-Sirve para expresar una crítica: "You could have bought another one for me."
-We use "could + inf" and " was/were allowed to + inf" to talk about
permission in the past. "Unfortunately, many years ago people could smoke anywhere". 

-We use "couldn't + inf" and "was/were not allowed to + inf" to talk about prohibition in the past. "British schoolchildren couldn't have long hair in the 1950's."