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

look for強調“找”的動作和過程。尋找一般的事物
He is looking for his dictionary bought yesterday.
他在找昨天買的那本詞典。

search for作“尋找；搜尋”解，其對象多為一個或一批人或物，如找礦，找資料，找文件，找工作等。它強調尋找的行為，不著重結果。
They are searching for the missing child．
他們在尋找失蹤的孩子。

think of
1.表示考慮到，想到，后常接名詞、代詞或動詞ing 形式
It's good to think of the future.
考慮到未來是對的
2.表示認為時，一般用于疑問句中，與what 連用
what do you think of the future?

think about

可接名詞、動詞ing 形式或由疑問句引導的不定式或賓語從句，表示“考慮……”
He's thinking about a problem.
他在考慮一個問題