The conditional ("if") form of "to be" is "were". "If I/he/she/it/you/they were..." "I was...", "He/she/it was...", "You/They/We were..." Note: If you say "I was...", I don't know of any native English speaker that would notice. Many of them say "If I was..." It is a very common mistake - common to the point where the only place you should really worry about it is on an English paper.

Was is singular and applies to one person. Were is plural and applies to more than one person. (except in singular "you were") For Example: 1. I was fit, but you three were not fit. 2. I was there yesterday. 3. If I were rich then I would quit my working occupation and travel all over the world

It's a remnant of a now defunct subjunctive form. Treat it as an exception. There aren't many of these.

when imagination is not related to reality then we used were after i.... eg: if i were a bird...... note that, its an impossible imagination....thats why here used were instead of was.

This is Subjunctive in English...and that's the form and structure of this kind of sentence,"If I/She/He/You were.....,...would/could/might"or"Were I/He/She/You...,...could/would/might"and some other kinds. Well...I just recognize that some native speakers use"was"instead of "were"..I don't know whether it's right or not,but you better use the word"were"..you can find many usage of "was"in American movies