Good morning, we'll talk about 以 today.
In our daily speaking, this character is mostly combined with other character to form words.
以为 think, consider, believe
我以为你不来了。 I thought you were not coming. (The fact is you did come.)
以为 and 认为
以为 is often used when one realizes that what one thinks is not true.
我以为我是对的。 I thought I was right. (Now I realize that I was wrong)
我认为我是对的。 I think I am right.