这些是书本例句，还是你自编的？

第1句错：

I'll never forget the day I joined the army. 这是最自然用法，如果要保留which，介词要用on, I'll never forget the day on which I joined the army. 

第2句错：时态混乱，你要么改时态this will be或其它，要么改用不定式。

改时态： That is the first time （that）you have visited China. 

   【错误】This is the first time you visit China. 

    【正确】This is the first time you have visited China. 

    【说明】在以 this / it / that is the first / second / third / only / best / worst 等构建的句子里，要用现在完成时。This is the first time that I've heard her sing.这是我第一次听见她唱歌。（不能说：This is the first time that I hear her sing.）（摘自《牛津英语用法指南》第三版460.2 和591节）

网友给的两个句子都有误，第一句in应为on；第二句visit应为have visited。另外，再给网友提个醒，“序数词+time”表示“第几次”，可用介词for，也可省略。

Mr. Smith just got married (for) the second time. 史密斯先生刚刚再婚。

网友给出英语句子需要更正为下面句子。

I will never forget the day when I joined the army

=I will never forget the day on which I joined the army

备注：具体在某1天，必须使用介词on, on which=when

This is the first time that you have visited China

=It is the first time that you have visited China

备注：that引导定语从句充当关系副词用法

“I'll never forget the day I joined the army. 这是最自然用法，如果要保留which，介词要用on, I'll never forget the day on which I joined the army. ”----（mikee）

---- 这句话很见功底！ 

【说明】在非正式文体中，表示时间的普通名词后面的关系副词 when 常常由关系副词that 来替代或省略。I'll never forget the day (that) we met. 我永远不会忘记我们见面的那一天。（摘自《牛津英语用法指南》第三版498.6节）----(my153)

但这样的句子，我们考试时却没法用。

学英语的人，注意这种地方非常有好处，

因为它涉及“语体”问题，而语体的平衡性，是让人一看，

就能探知对方英语的水深水浅软实力，因为它是个“形象工程”。