状语从句的时态问题

I would be grateful if you kept this news to yourself until he came back.

这是时间状语从句么，这里为何要用到came呢?

I would be grateful if you kept this news to yourself until he comes back.

这个句子不是“虚拟语气句”，而是表达“婉转请求”。

I would be grateful if ...

I would appreciate it if ...

until 从句，came 应改为comes。

退一步讲，即使 I would be grateful if you kept this news to yourself 这句话是虚拟语气，后面的时间从句 until hecomesback 也不受影响，仍然是陈述语气。比如：

I would give you a million dollarsif you kept this news to yourself until hecomesback.

虚拟语气句当中，含有陈述语气的从句是非常正常的。比如：在以下虚拟句中，各含一个陈述语气的从句，有定语从句和状语从句，主、从句中分别有一个。

If I saved a lot of money that I can earn, I would buy a BMW.

If I saved a lot of money when I grow up, I would buy a BMW.

If I saved a lot of money, I would buy a BMW when I grow up.

If I saved a lot of money, I would buy a BMW which is of real German make.

红色的从句，是客观存在的，它们不受虚拟语气的影响。