Marry was more thoughtful than I had imagined her to be.
1.为何这里是要用不定式? 并没imagine sb to do 这个结构阿,只有imagine sb doing吧?
2.这个比较句假如用imagined she was ,意思不同嘛?
《朗文》imagine
imagine somebody/something to be/do something
I had imagined her to be older than that.
所以你的句子中的than从句没问题的。