"It' you that I need"是英文中的强调句型,“It is(was)+被强调的部分+that(who)+原句其它部分”来强调说话人的意愿.这一句可以理解为第二句的变式,但是原句的谓语部分不能被强调(也就是说第二句可以将you和I分别强调,变为“It's you that I need”和“It’I that need you”,但不能将“need”用来强调).
第二句“I need you”是一般的陈述语句,简单陈述事实,无侧重
for example:All these things happened on Monday night(陈述语句)
It was on Monday night that all these things happened(强调语句 )