"They showed the new clothes."
Translation:Oni pokazali nowe ubrania.
Is there a reason it needs the "oni" here if you've used "pokazali?" Do pronouns have to be used more in the past tense or is this just a missed answer?
There is no need to use the pronoun here, however you can still do it. Generally, in written language people omit the pronouns in a normal sentence but in spoken language they use them more often.