Complex sentence: The airport was closed in 1955 when the new Singapore International Airport at Paya Lebar was built; although most of the airport was demolished and put to other use, the distinctive terminal building was used as the headquarters of the People's Association until April 2009.
Split into simpler sentences: The airport was closed in 1955 when the new Singapore International Airport at Paya Lebar was built. Although most of the airport was demolished and put to other use, the distinctive terminal building was used as the headquarters of the People's Association until April 2009.