Resumen.

1. Usaremos can y su negación can’t / cannot para expresar habilidad.

1.1. Oración afirmativa.

       Todd can surf well.

1.2. Oración negativa.

       Knox can’t swim well.

1.3. Oración Interrogativa.

       Can Neil play the drums?

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2. Could, and its negative form couldn't / could not are the past and conditional of can. Use it for:

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    a. Polite requests. → Could you open the door, please?

    b. Past ability. → My brother could run fast when he was 20.