20. They often had parties (on weekends).
⇒ It was on the weekend that they often had parties.
21. My brother bought his new car from our next-door neighbour (last Saturday).
⇒ It was my brother who bought his new car from our next- door neighbour last Saturday.
26. They went (to the cinema).
⇒ It was they went to the cinema last year.
27. We were supposed to meet (at 6 o’clock).
⇒ It was we were supposed to meet at 6 o’clock on Monday