If you really want to know WHY "If I were ..." is correct, you will have to examine the Subjunctive verb form. Try this website:
http://www.englishclub.com/grammar/verbs-subjunctive.htm
Here is a short excerpt from that site:
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Why do we say "I were", "he were"?
We sometimes hear things like "if I were you, I would go" or "if he were here, he would tell you". Normally, the past tense of the verb "to be" is: I was, he was. But [in FORMAL speech] the "if I were you" structure does not use the past simple tense of the verb "to be". It uses the past subjunctive ...
FORMAL: If I were younger, I would ...
FORMAL: If he weren't so mean, he would ...
FORMAL: I wish I weren't so slow!
INFORMAL: If I was younger, I would ...
INFORMAL: If he wasn't so mean, he would ...
INFORMAL: I wish I wasn't so slow!
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As noted, the "I were" structure is correct for formal / professional usage. However, in day-to-day average American conversation, you will hear "I was" more frequently.
Of course, the real reason WHY is ... 英文很神经病。