My attempt(s)-As soon as he completed his term, he was transferred.
Are you supposed to use "no sooner"? If so, then: No sooner had he completed his term than he was transferred. Past Simple would be fine as well, but Past Perfect puts more emphasis on the sequence of events.
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dimsumexpressIt is "than" not "then" used in this construction, and it is only correct with "had", not "didHi.