We can't use "could" in the following sentences:
- 2) We tried to open the door and were able to do it at last. (We could do it at last.)
- 3) They were not able to build a new garage. (They couldn't build a new garage.)
- 5) We didn't know the address, but we were able to find the hotel. (We couldn't find the hotel.)
- 7) That summer was very cold, but we were able to grow tomatoes in our garden. (We could grow tomatoes in our garden.)