We tend to pay little attention to the proper use of past perfect tense in everyday conversation and get away with simple past without much critique. We all knew Mandela had spent a major part of his life in confinement, 27 years to be exact, only to be released in 1990. This is one perfect example where past perfect should be used.
When there are two events linked together by time in the past, we should use Past Perf.
The duration of the preceding event takes Past P. simple past you are making a statement
|< his imprisonmentà| < his release| à the present time
