because+句子
because of+名词（短语）
I didn't go to school because I am ill.
I didn't go to school because of my illness.

look for强调“找”的动作和过程。寻找一般的事物
He is looking for his dictionary bought yesterday.
他在找昨天买的那本词典。

search for作“寻找；搜寻”解，其对象多为一个或一批人或物，如找矿，找资料，找文件，找工作等。它强调寻找的行为，不着重结果。
They are searching for the missing child．
他们在寻找失踪的孩子。

think of
1.表示考虑到，想到，后常接名词、代词或动词ing 形式
It's good to think of the future.
考虑到未来是对的
2.表示认为时，一般用于疑问句中，与what 连用
what do you think of the future?

think about

可接名词、动词ing 形式或由疑问句引导的不定式或宾语从句，表示“考虑……”
He's thinking about a problem.
他在考虑一个问题