| No | Pattern | Freq | Relations |
|---|---|---|---|
| 1 | it must [vb infin] be [prep] | 83 |
|
| 2 | must [vb infin] be [v-ing] [prep] | 81 |
|
| 3 | it must [vb infin] be [v-ing] | 22 |
|
| 4 | it must be [v-ing] [prep] | 10 |
|
