I started working at eight and I was still working at six in the evening, when you called. -> When you called, ten ?

Giải thích: ‘When you called’ là mốc thời gian trong QK. ‘I had been working for ten hours’ là hành động xảy ra trước, kéo dài tới mốc ‘when you called’ => sử dụng QKHT.
Dịch: ‘Tôi bắt đầu làm việc lúc 8 giờ và tôi vẫn đang làm việc lúc 6 giờ tối khi bạn gọi’

I started working at eight and I was still working at six in the evening, when you called. -> When you called, ________________________________ ten hours.

I started working at eight and I was still working at six in the evening, when you called.
-> When you called, ________________________________ ten hours.