on condition that 从句是不是得用虚拟语气

是的

不一定了，例如：

I'll come on condition that John is invited too.
如果约翰也受到邀请，我就来。

分析：

on condition that
adv.
如果

例句
You must on no condition climb that high wall.
你绝对不要爬那堵高墙。

On the understanding that...; Bon this understandingon condition that...; on this condition
在...条件下；以此为条件

I will lend you the book on condition that you return it on Monday.
如果说你能在星期一还我，我可以借给你这本书。

Capitulate on [upon] the condition that...
在。。。条件下投降