Hi Ilya
Great question! I hope my reply is useful to you. Happy studying!
If I (were|was) you, I'd be tired, too.
'If'.....so the situation is hypothetical, not real.
-> Past subjunctive of the verb to be = 'were'
Another example:
Let's assume that I am learning Russian. I am confused by a complex rule of Russian grammar.
Your reply to me might be "If I were you, I'd be confused too."