“既然那样”常见的英文表达有 “In that case” 或 “Given that”（后者语气稍正式些） ，具体使用示例如下：
In that case
If you can't come, in that case, we'll have to cancel the meeting.（如果你不能来，既然那样，我们就不得不取消会议了。）
You don't like this color? In that case, let's choose another one.（你不喜欢这个颜色？既然那样，咱们就选另一个吧。）
Given that
Given that you're not feeling well, you can go home early.（既然你感觉不舒服，你可以早点回家。）
Given that it's raining heavily, we'd better stay indoors.（既然雨下得这么大，我们最好待在室内。 ）