X. 

1. They went down the slope at great speed.

2. Because they were going very fast and it was too dangerous to use the brakes.

3. The bicycles crashed into a fence and a tree.

4. They were in great pain.

5. Their parents did.

XI. 

1. Mr and Mrs. Smit are travelling to Ha Noi now.

2. Van used to have a motorbike once.

3. My father usually drives to work.

4. This part of the city didn't use to have traffic jam.

5. I used to go to school on foot when I was young.

1. The bicycles went down the slope very fast (at great speed).

2. Because it was dangerous for them to use the brakes at high speed.

3. The bicycles crashed into a fence and a tree.

4. The boys were in great pain after the  crash.

5. Their parents came to bring them home after they was called.

XI,

1. Mr. and Mrs. Smith are travelling to Ha Noi now.

2. Van used to have a motorbike once but he doesn't any more.

3. My father usually drives car to work.

4. This part of this city didn't use to have traffic jams very often.

5. I used to go to school on foot when I was young.

Cho hay nhất ạ