"I wish I were" vs "I wish I was"

I wish I were is actually the preference of grammar experts because you’re talking about something that hasn’t actually occurred.

How to use I wish I were in a sentence

The subjunctive refers to words that describe doubtful situations—like wishes for things that aren’t real, that is wanted or desired but is not immediately possible.That’s why you’ll see I wish I were used in these examples:

I wish I were joking, but I really did crush the cake as it was coming out of the oven.

My mother was a great baker, and I wish I were more like her.

I wish I were older—I really want my own car!

 Why we still hear I wish I was

I wish I was a little bit taller

Well, let’s face it: some of our favorite songs are chock full of grammatical errors. These ’90s favorites aren’t exactly wrong. They’re simply non-standard.

While grammarians will tell you to stick to I wish I were to follow the rules of the subjunctive, language has evolved, and the non-standard I wish I was has become increasingly popular. Our advice?

If you’re looking to write a hit song, it’s fine to use the less formal I wish I was. If you’re writing a paper for your English professor, on the other hand, stick with the grammarians, and use I wish I were. (Thesaurus)

Alternatively, you should use the phrase “if I was…” when referring to something that actually happened. For example, 

“If I was late for curfew when I was a teenager, my parents grounded me.”

This variation is a lot less common, but it does come up.

A good trick to decide which you want to use is to determine if the thing you are talking about is something that actually happened or something that you are wishing or imagining might have happened. If it really happened, use “if I was,” but if not, go with “if I were.” (Reader's Digest)

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"I wish I were" is the most correct form, and, if you are writing a formal piece of writing, definitely go for it. No one can tell you that you are wrong.