I can not understand ______ the boy alone.A.why she left B.why she had left 请仔细说明原因

I can not understand you. I also can not ,but I can understand it what you said ,I can not understand是什么意思 I can not input chinese,you understand english? I can not understand why she left the boy alone . I also understand we can not have a love affair. I can not understand how you fee l Stone you,I really can not understand是什么意思 I can not understand这句什么意思 I sincerely to you!How can you not understand! I do not understand! I do not understand Can you understand?I Can you understand?I I do not want to leave you very.You understand?I can not you know? but I （could） not understand himbut I could not understand him.but I can not understand him.but I don't understand him.有何区别?是否可以互换?整句意思有何不同吗？ 翻译下这句话 I think I understand,Can not ask again why,Can not go to bother again May I join you?I hope what do not understand can ask you.