| No | Pattern | Freq | Relations |
|---|---|---|---|
| 1 | must [verb] been difficult | 12 |
|
| 2 | must have [be] difficult | 12 |
|
| 3 | must [vb infin] [be] difficult | 12 |
|
| 4 | must [verb] be difficult | 12 |
|
| 5 | must [verb] [be] difficult | 12 |
|
| 6 | must have [be] difficult | 12 |
|
| 7 | must [verb] [been] difficult | 12 |
|
