动词 is 是错的
正确的动词是 has
句子应该是
she has written for this magazine since 1996
she has written 也可以简写为 she's written
因此句子的另一个写法是
she's written for this magazine since 1996
但 she's 也是 she is 的简写
因此我想,句子或原来是
she's written for this magazine since 1996
被抄写错,误写为
she is written for this magazine since 1996