yes you can use عرف in i know something is ... like i know something is wrong (أعرف أن شيئا ما خاطئ) == you are sure that something is wrong for فكر you can say i'm thinking (أنا أفكر) but i think so is أظن(أعتقد) هذا so there is no فكر here i think it's true اعتقد(أظن) أن هذا صحيح the verbs اعتقد and اظن means you are not sure of something it's only in your opinion the verb think can means فكر too like in those examples i'm so busy to think about ... انا مشغول جدا لأفكر في