The passive voice version of "They built this house in 1990" is "This house was built in 1990". To change the sentence to passive voice, the object ("this house") becomes the subject, the verb changes to "was built" (the past tense of "to be" plus the past participle), and the original subject ("They") can be omitted or added after "by".
Change the voice: They built this house in 1990.
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