| No | Pattern | Freq | Relations |
|---|---|---|---|
| 1 | could have [verb] | 5379 |
|
| 2 | could have [vb pp] | 5343 |
|
| 3 | [modal v] have happened | 447 |
|
| 4 | could have happen | 107 |
|
| 5 | could [verb] happened | 107 |
|
