B. Cấu trúc chính của câu này: 
$S_{1}$ + $could$ + $V_{inf}$ + when + $S_{2}$ + $was/were$ + $O$ ...
(Trong trường hợp này nếu cả câu cùng nhắc đến một chủ ngữ thì ở $S_{2}$ có thể thay $S_{1} bằng ngôi thứ 3 số ít (he/she/it), không cần ghi lại tên được nhắc đến)
2. John could play piano when he was six.
3. My brother could draw beautiful pictures when he was seven.
4. We could speak two foreign languages when we were eighteen.
5. She could dance when she was eight.
6. Tiger Woods could play golf when he was three.
C. Cấu trúc chính của câu này:
$Could$ + $S_{1}$ + $V_{inf}$ + when + $S_{2}$ + $was/were$ + $O$ ... ?
(Trong trường hợp này nếu cả câu cùng nhắc đến một chủ ngữ thì ở $S_{2}$ có thể thay $S_{1} bằng ngôi thứ 3 số ít (he/she/it), không cần ghi lại tên được nhắc đến)
2. Could you write your name when you were three?
3. Could you read when you were two?
4. Could you count from 1-10 when you were two?
5. Could you talk when you were three?
6. Could you ride a bike when you were six?
7. Could you swim when you were five?
(Bài tập này không yêu cầu rõ chủ ngữ nên cố thể thay từ "you" trong câu bằng các ngôi khác, chẳng hạn như ngôi thứ 3 số ít (he/she/it), nhưng nếu thay đổi thì phải chú ý từ "was/were" sao cho phù hợp nhé)
Chúc bạn học tốt

$@Stormy$

$B.$ 

$2.$ John could play piano when he was six.

$3.$ My brother could draw beautiful pictures when he was seven.

$4.$ We could speak two foreign languages when we were eighteen.

$5.$ She could dance when she was eight.

$6.$ Tiger Woods could play golf when he was three.

$Structure :$

$⇒$ S + could + V_inf + O + when + S + was / were + age.

$⇒$ Cấu trúc trên diễn tả bạn có thể làm một việc gì đó trong quá khứ lúc mấy tuổi.

$C.$

$2.$ Could you write your name when you were three?

$3.$ Could you read when you were two?

$4.$ Could you count from 1-10 when you were two?

$5.$ Could you talk when you were three?

$6.$ Could you ride a bike when you were six?

$7.$ Could you swim when you were five?