Question

They had won the match.(change into passive voice)

Answer 1

"The match had been won by us" is the passive voice of "They had won the match".

When a sentence is changed from active to passive-

a. We should use the word " by".

b. We should change the verb to the third form of verb.

Thus, had won will change to had been.

Answer 2

the match had been won by them