Some people object to using a progressive form with "when", saying that one should use "while". However, I see nothing wrong with using "when" in such a case.
... when I was reading a report on the breeding of farm monkeys ...
... when reading a report on the breeding of farm monkeys ...
... while I was undertaking extra-curricular activities ...
... while undertaking extra-curricular activities ...
Please, post follow-up questions if you have any.
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