Can, Could and Be Able To — Ability and Requests
Practice can, could and be able to: present and past ability, will be able to, was able to for one success, and polite requests.
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My little sister is only four, but she ___ already swim.
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can
Modal verbs never take -s, even after 'she', and are followed by a verb without 'to': 'she can swim'.
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When I was five, I ___ read, but I couldn't write.
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could
'Could' is the past of 'can' for a general ability in the past ('when I was five'). 'Was able' needs 'to': 'was able to read'.
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Don't worry. When you finish the course, you ___ speak English much better.
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will be able to
Two modals can't go together, so 'will can' is impossible. For future ability, use 'will be able to'.
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___ you open the window, please? It's really hot in here.
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Could
'Could you…?' is a polite request. 'Do you open the window?' asks about a habit, not for help.
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I'm sorry, I ___ come to your party on Saturday — I'm working.
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can't
Working makes it impossible → 'can't'. 'Mustn't' means somebody forbids it, and 'don't have to' means it isn't necessary.
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The test was really hard, but in the end I ___ pass it.
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was able to
For one specific success in the past, use 'was able to' (or 'managed to'), not 'could'. 'Could' is for a general past ability: 'I could swim when I was six.'
Can, Could and Be Able To
Can expresses ability and permission in the present: My sister can swim, Guests can use the pool. Could is its past form for a general ability — When I was five, I could read — and the polite form for requests: Could you open the window, please? Because can is a modal, it has no future form and cannot follow will; for future ability English uses will be able to: After the course, you'll be able to speak much better. The subtler point is the past. For one specific success on one occasion, English uses was able to or managed to, not could: The test was hard, but I was able to pass it. Could describes what someone was generally capable of, not what they achieved once. In the negative the distinction disappears, and couldn't works for both.
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