The passive voice of modal auxiliaries is formed by the modal + be + past participle. This formation can be used to express the present and future tenses.
- The door can’t be opened.
- Children should be taught how to read poetry.
- This package ought to be sent by tomorrow.
- Fred has to be told about the meeting.
- Jason was supposed to be informed about the changes.
The future tense
With some modals, the future tense is expressed with the auxiliary will.
- Fred will have to be told about the meeting.
The past tense
The past tense of certain modal auxiliaries in the passive voice is formed by the modal + have been + past participle.
- The letter should have been sent yesterday!
- This car must have been stolen two months ago.
- Andrew ought to have been told about the meeting.
With other modal auxiliaries, the past tense of the modal is used together with to be + past participle.
- Fred had to be told about the meeting.
- The door couldn’t be opened.