Sure! Here are the completed sentences along with explanations for each choice:

2. When Dan was 16, he **could** run 100 metres in 11 seconds.
- **Could** is used here to indicate Dan's past ability to run fast.

3. ‘Are you in a hurry?’ ‘No, I’ve got plenty of time. I **can wait**.’
- **Can** is used here to express the present ability or possibility. Since he has plenty of time, he is able to wait.

4. I don’t feel good this morning. I **couldn’t** sleep last night.
- **Couldn't** is used here to express a past inability. Since the person didn't sleep well, they are feeling unwell this morning.

5. Can you speak a little louder? I **can’t hear** you very well.
- **Can’t** is used here to express the present inability to hear clearly.

6. I was amazed when I heard the news. I **couldn’t believe** it.
- **Couldn't** is used here to express a past inability to believe something surprising.

I hope this explanation helps you understand the usage of can/can't/could/couldn't in these sentences! If you have any more questions, feel free to ask.