It is difficult for us to learn a lesson in life until we have actually had that lesson.这句话的从句中为什么要用现在完成时呢,前面用的不是一般现在时吗?

it is very difficult for me. it is difficult a book for us it is difficult for me.为什么要用for?是it is too difficult for me. it's difficult for math is difficult for me.=it's difficult for me （）（）math. I thought it( ) me to work out the problem.A.difficult for B.is difficult for because it is difficult （）me 填to还是for it,very,difficult,to,is,well,learn,english,him,for It is a too difficult book for beginners对吗 It is very difficult for her______(climb) up the mountain. It is a difficult book for beginners对吗应该怎麼改？ It is difficult for you to speak English是什么意思 it is difficult it is difficult at it is so difficult for sb to do sth,difficult用so形容对吗? 是it is difficult to me to do sth还是it is difficult to me for sth还是it is difficult to me to sth Things difficult for a woman to do it more difficult to do man is difficult! 翻译下 I think that it is _____ problem for us.a quiet difficult b quiet a difficult c very difficult a