These questions ask you to choose an ability expression that fits both the time frame and the kind of ability being described. A reliable choice comes from reading the whole situation before checking the grammar of the verb phrase.
Read the time and the result
Start by locating the action in the present, past or future. If it is past, make a second decision: is this an enduring or repeated skill, a perception or thought, a failed attempt, or one action that the person actually completed? That distinction matters more than the mere presence of a past-time expression. Finally, check whether the surrounding grammar needs an infinitive or another form that can and could do not possess.
| Signal in the sentence | Ability form to consider |
|---|---|
| A skill or capacity that is true now | Use can followed by the bare verb; is or are able to can also express present ability. |
| A skill held over a period or used repeatedly in the past | Use could or the past form of be able to. |
| One affirmative action that was successfully completed | Use was or were able to, or managed to, rather than affirmative could. |
| Past perception or cognition | Could remains natural with verbs such as see, hear and understand. |
| A failed action in the past | Could not and the negative past form of be able to can both report the failure. |
| A future, infinitive or perfect construction | Use the required form of be able to because can has no future, infinitive or participle form. |
After choosing the meaning, perform a short form check. Can and could are followed directly by the bare verb. They never gain an -s ending, even after he, she or it, and they do not follow another modal. Be able to behaves differently: be changes to fit the construction, while able to stays together.
The traps
| Tempting form | How to reject it |
|---|---|
| Adding -s to can for a third-person subject | A core modal never changes for person or number. Keep can unchanged and put the following verb in its bare form. |
| Putting to between can or could and the main verb | The modal already carries the grammatical work, so the next verb is bare. To belongs inside be able to, not after can or could. |
| Placing can or could after will | Two core modals cannot be stacked. When the time is future, let will carry that meaning and follow it with be able to. |
| Choosing affirmative could simply because an event happened in the past | Check the result. An enduring ability permits could, but a single action explicitly completed successfully calls for was or were able to, or managed to. |
| Forcing can into an infinitive position | Can has no infinitive. After a verb that requires to plus an infinitive, use to be able to. |
Try it
Complete the achieved-action sentence with a multi-word ability phrase: After the upload failed twice, Elena tried a different network and ___ the portfolio before the deadline.
Complete the sentence with could or the past form of be able to, using the exact phrase in brackets: Throughout his years as a field geologist, Omar ___ without a compass. (read maps)
Complete the negative one-off sentence with could or be able to, using the exact verb in brackets: Despite using the correct code yesterday, Sofia ___ the storage locker. (unlock)