If I were... Subjunctive mood.

When you grow up with a language you just know some things, without having to think about it. When you learn a new language, it's different. 

My English teachers way back when drilled me with tables like:

I  am

You  are

He, She, It  is

We  are

You  are

They  are

And

I  was

You  were

He, She, It  was

We  were

You  were

They  were

After a while it sticks and becomes natural.

So, why do I have such an urge to say, "If I were" or, "If she were?" According to the tables it should clearly be was, not were. Right?

Wrong.

My "were" in these types of sentences have been corrected by many editors in different situations. When it comes to fiction I generally go along with it. It's not that big a deal. However, the word were is the right form to use together with if. 

It shows a wish or thought that isn't true; a statement that doesn't form an established fact. It can also be about someone's state of mind, opinion, purpose, belief, desire, or intention. This is called subjunctive form.

For example, saying, "If he were a blue elephant" is correct, because the were indicates that something is out of the ordinary and not true.

Here are a couple of examples in present and past form.

Present indicative			Present subjunctive
I	own		I	own
You	own		You	own
He, She, It	owns		He, She, It	own
We	own		We	own
You	own		You	own
They	own		They	own
Past indicative			Past subjunctive
I	was		I	were
You	were		You	were
He, She, It	was		He, She, It	were
We	were		We	were
You	were		You	were
They	were		They	were

If you want to delve into this, here are some great resources:

• The Oxford Dictionaries have a great explanation.
• The Language Log elaborates on, "The 'were' form is often wrongly called a past subjunctive, but of course 'it were done' is not a past tense of 'it be done'.
• Englishclub.com also elaborates on the subjunctive.