past continuous & past perfect continuous

past continuous tense通常與simple past tense一起用，例如：
I was watching TV when John arrived at home.
句中有兩樣事情發生，John回到家中是我正在看電視的時候發生的．換句話說，若一件事進行期間有另一件事發生，原先已在進行的便用past continuous tense.

past perfect continuous通常也涉及兩件事，再以上述句子為例，若用past continuous tense，作者便沒有交代John回家後我是否繼續看電視，但假如用的是past perfect continuous tense, ie,
I had been watching TV when John arrived at home.
意思便是John回家後我繼續看電視．