by the time主从句时态

这个词组常用来指到某一点时间为止,主句常用完成时态.具体如下：
1）从句用一般现在时,主句用将来完成时（will have done).
Bythe time I graduate next year,I will have lived here for 5 years.到明年毕业时,我就在这里呆了五年了.
2）从句用一般过去时,主句用过去完成时.
e.g.Bythe time he was 12,he had traveled to more than 30 countries to get his diseases cured.到12岁为止,他已去过三十多个国家治病.