I saw this sentence on TV:

日々心によぎる「あのとき こうしていれば」という言葉。。。

and I wonder why していれば is used here and not すれば.

When I asked my teacher he said it is an N1 grammar point, he tried to explain it to me but I didn't get it.

I'm looking for more info on this to understand why it is like that, I didn't find it on a quick google search.

Here is one more example sentence that he gave me:


The suggested duplicate questions don't answer why it is like that, they just state that this is the right way.
I still don't know why they are using ていれば, if it is just like that - fine. But if there is a reason for a western mind to understand I will be glad to hear it.

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