We looked for them,but we could not find them all.为什么用find?
新概念一册第117课：
1、We looked for them,but we could not find them all.为什么用can的过去式,而又为什么用find?能不能说成but we can't found them all.
以下这句为什么用found?
2、He looked for his pen everywhere in the room,but he hasn't found it yet.
谁能说下这两句的曲别在哪,第一句作者想要强调什么,为什么用could和find,第二句作者又是想要强调什么,为什么用found?
第一句到底是什么句型？