Why would he say "If I were you" even though the word "I" refers singular noun?
This is a great question! Generally, we might think that "I was you" would be more appropriate considering "I" corresponds with "was." However, when trying to express a subjunctive mood, "If I were you" is almost always used instead of "If I was you." Ex: If I were you, I wouldn't go out today. It's snowing really heavily. Ex: I'm so envious you live in California. If I were you, I would go to the beach all the time.