| Positive |
|||
| Modal | Past | Future | |
| 1a | can | could | cannot use can use rule 2a |
| 2a | to be able to | was/were able to | will be able to |
| 3a | to know how to | knew how to | will know how to |
| Negative | |||
| Modal | Past | Future | |
| 1b | cannot | could not | cannot use can use rule 2b |
| 2b | to (not) be able to | was/were not able to | will not be able to |
| 3b | to (not) know how to | did not know how to | will not know how to |
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