3 Discuss why different modals are used in these pairs of examples.
1 He could swim when he was six.
When the boat sank, he was able to swim to shore.
I must buy a new suit. This one's looking scruffy.
I have to buy a suit for my new job.
3 You should say thank you.
2
You could at least say thank you!
4 My dad would often make up stories for us.
My dad used to be slim when he was young.
5 We didn't need to take our own towels.
We needn't have taken our own towels.

1. In the first pair of examples, "could" is used to describe past ability, while "able to" is used to describe a specific event in the past where the person was able to swim to shore. "Could" refers to a general ability or skill, while "able to" is more specific to a particular situation.

2. "Must" is used to express an obligation or necessity, while "have to" is used to express a requirement or obligation. In the first sentence, the speaker expresses a personal desire to buy a new suit, while in the second sentence, there is a specific obligation to buy a suit for a new job.

3. "Should" is used to give advice or suggestions, while "could at least" is used to express frustration or disappointment with someone's behavior.

4. "Would" is used to describe a habitual action or behavior in the past, while "used to" is also used to describe a past habit or behavior that has now changed.

5. "Didn't need to" is used to describe a situation where something was not necessary, while "needn't have taken" is used to describe a situation where something was done unnecessarily. The first sentence suggests that taking towels was not necessary, while the second sentence suggests that the speaker regrets taking the towels.