为什么是thirty years ago 而不是thirty years before?

为什么是thirty years ago 而不是thirty years before? Thirty years or so ago什么意思? How was Shenzhen thirty years ago?意思 He has joined the party thirty years ago.其中,has ,joined party,thirty years ago分别为ABCjoin不是瞬间动词吗，怎么一个回答也不提。 为什么是since two years ago 而不是for two years ago Thirty five years ago和Thirty five years before有什么区别?不解.. It was three years ago ( ) he left 为什么是that? 择题When did you decide to be a teacher? Let me see.I am thirty,and I have wanted to be one since_a twenty years b twenty years ago c I had twenty years d I was twenty答案为什么是dsince+一段时间+ago为什么不可以？ Tom left his home thirty years ago 同义句转换 my grandpa joined the party thirty years ago 改同一句my grandpa___,__-,___ the party for thirty years My father joinde the Party thirty years ago.同意转换My father___ ___ ___the Party for thirty years Tom married Kate thirty years ago.(改为同义句)Tom___ ____ ____ ____ Kate for thirty years . thirty years on 是什么意思?---------------------------------------|-------------|--------|-------------|--------时间轴 50年前 30年前 20年前 现在 50 years ago,food deliveries was…… thirty years on ,foo he drives much (care)___than he did 3 years ago.空里为什么是more carefully how many people ____there thirty years ago 1.will 2.were 3.are 4.will be Thousands of people _______ the earthquake thirty years ago (die) 要原因 These shoes cost (thirty yuan)two years ago.括号部分提问 Over thirty years ago he left his hometown .Over 可以换成什么