第三十九天

[nnozomi]

是...的 to emphasize a specific point
https://resources.allsetlearning.com/chinese/grammar/The_%22shi..._de%22_construction_for_emphasizing_details

Let's stick to the easy half of this (as it strikes me) today and move on to the use in past questions tomorrow...

Guardian: (ugh, this is impossible to search for without knowing more about 正规表现 than I do)
我们都是从地星来的。(it's from Dixing that we all came)
让是你来指认现场的，不是让你去打先锋的 (it's to confirm the site that you're here, not to go strike the first blow) (?)
但是强大是要付出代价的 (but strength requires paying the price)

My practice:
我是坐地铁来的，走路太远了。
昨晚你原来是跟他去玩的！

Comments

[nnozomi]

But to speculate anyway, I think I would have taken 是 as the main verb in both sentences, and the 是...的 construction as basically forming an adjective that gives you information about the subject of the sentence.
On reflection, that makes more sense. 谢谢！ Oh dear.

[grayswandir]

Oh dear.

That's kind of where I'm at. XD