关于英语语法的问题,前半句用了完成时,后半句还能用完成时吗?还是用过去式?

I havent started yet,cuz I havent prepare for it 可以的.关键是看时间状语.
再问： I didnt start,cuz I havent ready for it yet 这个说法也可以吗？？ 时间状语。你给举个例子有时间状语的！
再答： 就是根据后面加的时间状语么，比如说I havent started yet,cuz I didn't prepare yesterday. yesterday 就限定了过去式了，很简单的。再比如如果I did not start yesterday, cuz I had not prepared for it by then. 后面一句是到昨天为止的过去完成时，所以都是看时间状语来限定的。
再问： 饿，还是不怎么理解啊。。。那按照你这样说就不能说成是I havent started yet,cuz I didn't prepare yesterday了咯，那只能用后句I did not start yesterday, cuz I had not prepared for it by then。那么这句的时间状语是什么？然后有时间状语就用一般过去，没有时间状语就用过去完成时，或者现在完成时？