$\text{@su}$
`1`, I could play chess when I was six
`2`, We couldn't play football because it was raining
`3`, I said she cold not understand anything.
`4`, When you were five, could you speak English?
`5`, When my brother was two he could draw pictures
hoặc My brother could draw pictures when he was two.
`6`, Could the robots clean the house ten years ago?
`7`, This suitcase was very heavy so I couldn't lift it.
`8`, Dave could play tennis well when he was young.
hoặc When Dave was young he could play tennis
Bonus: S + could + (not) + Vbare + when + S + was/were + ...
$\text{#BTS}$