could

Used as the past form of can to talk about what someone was able to do.

• She could speak three languages fluently.
  Üç dili akıcı bir şekilde konuşabiliyordu.
• I looked everywhere but couldn't find my keys.
  Her yere baktım ama anahtarlarımı bulamadım.

Used to say that something is possible now or in the future.

• Take a coat; it could rain later today.
  Mont al; bugün ilerleyen saatlerde yağmur yağabilir.
• Be careful; that dog could be dangerous.
  Dikkatli ol; o köpek tehlikeli olabilir.

Used to ask for something or to ask permission in a polite way.

• Could you please help me with this?
  Bana bu konuda yardım edebilir misiniz lütfen?
• Could I use your phone for a minute?
  Bir dakikalığına telefonunuzu kullanabilir miyim?

Used to suggest something that someone can do.

• We could go to the cinema tonight if you like.
  İstersen bu akşam sinemaya gidebiliriz.
• You could ask your sister to help you.
  Kız kardeşinden yardım isteyebilirsin.

Used in conditional sentences to say what would be possible.

• If I had more money, I could buy a new car.
  Daha çok param olsa yeni bir araba alabilirdim.
• We could go for a walk if it stopped raining.
  Yağmur dursa yürüyüşe çıkabilirdik.

Used in the form could have done for something that was possible but did not happen.

• We could have won the match if we had played better.
  Daha iyi oynasaydık maçı kazanabilirdik.
• You could have told me you were coming!
  Geleceğini bana söyleyebilirdin!

Used in the form could have done to say that something possibly happened.

• He's late; he could have missed the bus.
  Gecikti; otobüsü kaçırmış olabilir.
• Someone could have taken your umbrella by mistake.
  Biri şemsiyeni yanlışlıkla almış olabilir.

Used as the past of can for permission, especially when reporting what someone said.

• She said that I could borrow her car.
  Arabasını ödünç alabileceğimi söyledi.
• When I was young, I could stay up late at weekends.
  Küçükken hafta sonları geç yatabiliyordum.