Question

Put into passive voice Did they stop the train at once?

Answer 1

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Was the train stopped by them at once ?

Structure ↴

Auxiliary / Be verb + Object + Past participle form of the main verb + by + Subject + question mark (?)

Explanation ↴

The given sentence is a normal structure of interrogative sentence in active voice. To turn an active sentence starting with 'Do', 'Does', 'Did', into a passive one, 'Am', 'Is', 'Are', 'Was', and 'Were' are placed according to the subject instead of 'Do', 'Does', and 'Did'.

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Answer 2

Aɴsᴡᴇʀ :-

Was the trained Stopped by them at once.