Sure, let's complete each group of sentences with the appropriate verb from the list provided. Here are the verbs and their meanings:

1. **bring down**: to reduce or lower something.
2. **come down**: to descend or fall.
3. **go down**: to decrease or be reduced.
4. **hold down**: to keep something at a low level.
5. **knock down**: to demolish or hit someone/something to the ground.
6. **put down**: to place something on a surface or to euthanize an animal.
7. **take down**: to remove something from a higher position.
8. **turn down**: to reject or refuse something.

Now, let's fill in the sentences:

1. We'll have to **knock down** that wall.
Two witnesses saw him **knock down** the pedestrian.
If you **bring down** the price, you'll sell them all.
How could you **turn down** such an offer?
We will, of course, **turn down** any unsuitable applicants.

2. Shall I **turn down** the volume?

3. How much must I **put down** as a deposit?
Vets hate having to **put down** pets - even if they are injured.
He tends to **put down** who doesn't agree with him.

4. They're fighting to **bring down** the government.
The new policy will **bring down** prices.
We saw the defender **bring down** their centre forward.

5. It's time to **take down** the Christmas decorations.
My secretary will **take down** the details.
Can you help me to **take down** the tent?

6. He can't **hold down** a job for very long.
We've managed to **hold down** prices.
It took four men to **hold down** such a strong person.

7. You must **come down** to the country and visit us.
The fog has **come down** and made driving dangerous.
We expect them to **come down** in favour of the proposal.

8. The swelling will **go down** in a few days.
I want him to **go down** on his knees and apologise!
The news didn't **go down** very well.

These sentences now correctly use the verbs from the list provided.