Penser : 觉得, 以为, 认为

觉得 (juéde) : Expresses a personal sentiment, impression, or subjective opinion.
Ex: 我觉得这件衣服很漂亮。 (wǒ juéde zhè jiàn yīfu hěn piàoliang.) (I think this piece of clothing is pretty).

以为 (yǐwéi) : "Believe wrongly". Often used when one has mistaken a situation.
Ex: 我以为你不在家。 (wǒ yǐwéi nǐ bù zàijiā.) (I thought you were not at home).

认为 (rènwéi) : More formal. Expresses a reflective judgment, conviction, or argued opinion.
Ex: 我不认为这是个好主意。 (wǒ bù rènwéi zhè shì ge hǎo zhǔyi.) (I do not consider it a good idea).