He can/ is able to cook really well because he used to work in a restaurant.

2)To work for this airline you must can/must be able to speak English fluently.

3)I’m really sorry I couldn’t/ wasn’t able to come to your party last Saturday.

4)If it doesn’t rain, we can/ we’ll be able to go to the mountains tomorrow.

5)I’ve been so busy I haven’t could/ haven’t been able to call him yet.

6)I used to can/ used to be able to speak a bit of Russian but I’ve forgotten it now.

7)If we had a bit more money, we could/ would be able to buy a better car.

8)I hate not can/ not being able to tell my boss what I really think of him!

9)He could/ was able to talk whan he was only 18 months old.  - Здесь не ясно без форматирования. Если to стоит отдельно, то выбираем was able, так как to не употребляется после could.

10) Can you/ Will you be able to help me tomorrow?  - то же самое, если to не выделено тем же шрифтом что и will you be able, выбираем will you be able, в противном случае оба варианта правильны.  

11) I’ve never could/ never been able to dance very well.

12) To live in the country you need to can/ be able to drive.