過去にできたことを言うときは、could ＋ 動詞の原形を使います。could のあとに to はつけません：I could to swim ではなく I could swim です。
could ＋ 動詞の原形
When Amina was five, she could swim.
Hiroshi could ride a bike at six.
I could read when I was four.
Not "She could to swim." — say "She could swim.". could と動詞の間には何も挟みません。to は不要です。
やってみよう: When Amina was five, she ___. (could swim / could to swim / could swimming) 答え: could swim — could のあとには動詞の原形をそのまま使います：could swim。could のあとに "to" や "-ing" はつきません。
誰が主語でも同じ（-s は不要）
could は形が変わりません。he, she, it でも could を使い、そのあとの動詞にも -s はつきません。
I could speak English.
She could speak English.
They could speak English.
Not "She could speaks English." — say "She could speak English.". 主語が she でも動詞は原形のままであり、could speaks にはなりません。
やってみよう: When she was young, Sofia ___ French. (could speak / could speaks / coulds speak) 答え: could speak — could に -s はつきません。またそのあとの動詞も原形のままです：could speak（"could speaks" や "coulds" にはなりません）。
否定文：couldn't
できなかったことを言うときは、couldn't ＋ 動詞の原形を使います。（couldn't ＝ could not です。）
She couldn't find her keys.
I couldn't swim when I was three.
They couldn't come yesterday.
Not "She couldn't found her keys." — say "She couldn't find her keys.". couldn't のあとでは、動詞は原形になります：found ではなく find です。
やってみよう: Hiroshi ___ his keys this morning. (couldn't find / couldn't found / didn't could find) 答え: couldn't find — 否定文は couldn't ＋ 動詞の原形です：couldn't find（"couldn't found" や "didn't could" にはなりません）。
疑問文：Could を文頭に
質問するときは、主語の前に Could を置き、そのあとに動詞の原形を続けます。
Could you swim at five?
Could she speak French?
What could you do?
Not "Did you could swim?" — say "Could you swim?". 疑問文では Could を文頭に置きます。'did' は不要です。
やってみよう: ___ you ride a bike when you were six? (Could / Did could / Were) 答え: Could — 疑問文は主語の前に Could を置いて始めます：Could you ride a bike?。could の前に "did" を足す必要はありません。
やってみよう: Remember what follows could, As a child, Lani ___ a bike. (could ride / could rides / could to ride) 答え: could ride — could のあとは常に動詞の原形です：could ride。（-s も "to" も "-ing" も一切つきません。）