If I were in your shoes informal
Used to give an example of what you would do in someone's situation
If I were in your shoes, I wouldn't waste money on shopping online.
If I were in your shoes, I would study hard to get the scholarship this semester.
"if someone were in your shoes" is if Clause or Conditional Sentence Type 2, so "someone would + (verb)" usually goes after "if someone were in your shoes".
In the 1700s, they used condition clauses with "if", the word "Stead" in the 1300s, and "Place" from the 1500s.
To do domething based on your emotions rather than for rational reasons
I can't believe that Tom beat his classmate. He's always the one who let his heart rule his head.