Complex sentence: It was formed in the 1950s from the merger of the Singapore Progressive Party (SPP) and the Democratic Party (DP), the latter not to be confused with the Singapore Democratic Party (SDP).
Split into simpler sentences: It was formed from the merger of the Singapore Progressive Party (SPP) and the Democratic Party (DP) in the 1950s. The DP is not to be confused with the Singapore Democratic Party (SDP).