Modal verbs: could

This is the second post in my modal verb series. You might want to read this post about "can" first. "Could" is often used as the past form of "can" but this isn't the only meaning. Keep reading to learn more.

We can use "could" to talk about:
​1. Ability in the past
I could run faster when I was young.
He could speak two languages before he was five years old.
2. Possibility in the past
We could buy a lot more for 10p in 1970!
He couldn't buy any milk because the shops were closed.
Note:
could + have + verb 3 means something was possible but didn't happen. For example:
We could have met for coffee if I had know you were in town.
3. Hypothetical possibility
I'd read more if I could find time.
It could be a long time before we know what really happened.
4. Rules and permission in the past
We couldn't wear jeans at school.
When I was a child, I could stay up later at the weekend.
5. Asking politely
Could you help me with this, please?
Could I borrow some money?
6. Speculation
He isn't answering the phone. He could be in a meeting.
I guess he could be about 50 years old. I'm not sure.
7. Reported speech
He said, "I can cook dinner."
He said that he could cook dinner. 

Could vs was/were able to:
​To talk about skills and abilities in the past, we can use "could" or "was/were able to". The meaning is the same but "was/were able to" can sometimes sound more formal than "could". For example:
He could swim. ✔️
He was able to swim. ✔️

When we are talking about the ability to achieve something on one specific occasion, however, there is a small problem that confuses a lot of students. "Could" is not possible here.
I was able to get some tickets before they sold out. ✔️
I could get some tickets before the sold out. ❌
You could also say:
I managed to get some tickets. ✔️
I succeeded in getting some tickets. ✔️ (Notice the -ing form here.) 

With verbs for senses, see, hear, smell, taste and feel, both forms are possible.
We were able to see the sea from the hotel window. ✔️
We could see the sea from the hotel window. ✔️

In a negative sentence, this problem disappears and we can use either form.
I wasn't able to get any tickets. ✔️
I couldn't get any tickets. ✔️