我以为今天会下雨，所以带了雨伞。

Wǒ yǐwéi jīntiān huì xiàyǔ, suǒyǐ dài le yǔsǎn.

I think it will rain today, so I brought an umbrella.

我本来以为我的英语很好，但是到了美国以后，我发现我错了，我听不懂他们说的英语。

Wǒ běnlái yǐwéi wǒ de Yīngyǔ hěn hǎo, dànshì dào le Měiguó yǐhòu, wǒ fāxiàn wǒ cuò le, wǒ tīngbudǒng tāmen shuō de Yīngyǔ.

I originally thought my English was good, but after arriving in The States, I found out I was wrong. I don't understand their English.

他以为这个问题很简单。

Tā yǐwéi zhège wèntí hěn jiǎndān.

He thought this question was very easy.