we just couldn't________（本没想到你能准时到达）,because we had been informed of the terrible jam.

完全正确答案：
we just couldn't_have imagined you would arrive on time____（本没想到你能准时到达）,because we had been informed of the terrible jam.为什么不就用：imagine 呢，本没想到你能准时到达，但你却准时到了。还是指我没想到过，过去没能不就用couldn't do 吗？couldn't have done 意为不可能做了。。。这是基于对当时发生情况的理解，即本不可能想到，实际上却想到了。现在再想起来，实际上我当时想得过于复杂了，实际上重点应放在（实际发生的）估计你居然能准时到达，而原来以为你是不可能准时到的。估计你们老师也没期望得到那么难的句子。因此，现改为你们老师期待的简单正确答案。同时，建议你告诉老师because前面应去掉逗号。 We had not imagined you would arrive on time____（本没想到你能准时到达）because we had been informed of the terrible jam.