past perfect con.

[5jj]

I was studying English from 2 to 4 p.m. yesterday. (it is ok)

I left early. I got tired because I was skiing from 2 to 4 pm.
(It is not ok...We have to use past perfect cont. tense.) Some people would use 'was skiing.' When the sequence of actions is clear, native speakers are not always as concerned about using past perfect forms as some grammar books suggest.

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