Know that I love you 不知道为什么也要加that?

Know that I love you 不知道为什么也要加that? 有首歌里有句everyone know that i love you Always know that I love you so I don't know that you love you know that I will love you tall I you know that I will love you till I die . 英译汉短文!I know that you love me,and i hope you know that i car You have to know that I love you Darling,you know that I have so much love you? Dear,You should know that I really love you Let you know that I love you 翻译汉文 just wanted to let you know that I LOVE YOU