This problem was much more difficult than i ---- (expect).

This problem 【was信息词】 much more difficult than I【had expected】 (expect).
句子意思：
这个问题比我当时估计的难得多.
信息词：
was一般过去时态
句子意思对比：
比【此前估计】的难得多,表明【估计】这个动作发生在【was】之前,即要用【过去完成时态】.
This problem 【is 】much more difficult than I【expected】.
祝你开心如意!